Forty Seven (FTSV) 424B5 Prospectus supplement for primary offering

Filed Pursuant to Rule 424(b)(5)
Registration No. 333-232498

PROSPECTUS SUPPLEMENT

(To Prospectus dated July 12, 2019)

9,375,000 Shares

COMMON STOCK

We are offering 9,375,000 shares of our common stock pursuant to this prospectus supplement. Our common stock is listed on The Nasdaq Global Select Market under the symbol “FTSV.” On July 17, 2019, the closing price of our common stock on The Nasdaq Global Select Market was $8.76 per share.

We are an “emerging growth company” as defined under the federal securities laws. Investing in our common stock involves risks. See “Risk Factors” on pageS-15 of this prospectus supplement and on page 6 of the accompanying prospectus and in the documents incorporated by reference into this prospectus supplement and the accompanying prospectus.

PRICE$8.00 A SHARE

	Price to Public	Underwriting Discounts and Commissions(1)	Proceeds to Forty Seven
Per share	$8.00	$0.48	$7.52
Total	$75,000,000	$4,500,000	$70,500,000

(1) See “Underwriters” for a description of the compensation payable to the underwriters.

We have granted the underwriters an option to purchase up to an additional 1,406,250 shares of common stock at the public offering price less underwriting discounts and commissions.

A member of our board of directors and his affiliated entity, as well as an existing stockholder and a strategic collaborator that had submitted indications of interest, have agreed to purchase an aggregate of 1,750,000 shares of our common stock in this offering at the public offering price.

The Securities and Exchange Commission and state securities regulators have not approved or disapproved of these securities, or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The underwriters expect to deliver the shares of common stock to purchasers on July 22, 2019.

MORGAN STANLEY	CREDIT SUISSE
GUGGENHEIM SECURITIES	CANACCORD GENUITY
H.C. WAINWRIGHT & CO.	ROTH CAPITAL PARTNERS

July 17, 2019

TABLE OF CONTENTS

Prospectus Supplement	Page
About This Prospectus Supplement		S-i
Prospectus Supplement Summary		S-1
Risk Factors		S-15
Special Note Regarding Forward-Looking Statements		S-16
Industry and Market Data		S-18
Use of Proceeds		S-19
Dividend Policy		S-20
Capitalization		S-21
Dilution		S-22
Material United States Federal Income Tax Consequences ToNon-U.S. Holders of Our Common Stock		S-23
Underwriters		S-27
Legal Matters		S-33
Experts		S-33
Where You Can Find Additional Information		S-33
Incorporation of Certain Information By Reference		S-34

Prospectus	Page
About This Prospectus		i
Prospectus Summary		1
Risk Factors		6
Special Note Regarding Forward-Looking Statements		7
Use of Proceeds		9
Description of Capital Stock		10
Description of Debt Securities		17
Description of Warrants		24
Legal Ownership of Securities		26
Plan of Distribution		30
Legal Matters		32
Experts		32
Where You Can Find More Information		32
Incorporation of Certain Information By Reference		33

ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering and adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference herein. The second part, the accompanying prospectus, provides more general information. Generally, when we refer to this prospectus, we are referring to both parts of this document combined. To the extent there is a conflict between the information contained in this prospectus supplement and the information contained in the accompanying prospectus or any document incorporated by reference therein filed prior to the date of this prospectus supplement, you should rely on the information in this prospectus supplement; provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date—for example, a document incorporated by reference in the accompanying prospectus—the statement in the document having the later date modifies or supersedes the earlier statement.

We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference herein were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

Neither we nor the underwriters have authorized anyone to provide any information other than that contained or incorporated by reference in this prospectus supplement, in the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. Neither we nor the underwriters take any responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you.

This prospectus supplement, the accompanying prospectus and any such free writing prospectus do not constitute an offer to sell, or a solicitation of an offer to purchase, the securities offered by this prospectus supplement, the accompanying prospectus and any such free writing prospectus in any jurisdiction to or from any person to whom or from whom it is unlawful to make such offer or solicitation of an offer in such jurisdiction. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and in any free writing prospectus that we have authorized for use in connection with this offering is accurate only as of the date of those respective documents. It is important for you to read and consider all information contained in this prospectus supplement and in the accompanying prospectus, including the documents incorporated by reference herein and therein, in making your investment decision. You should also read and consider the information in the documents to which we have referred you in the sections entitled “Where You Can Find Additional Information” and “Incorporation by Reference” in this prospectus supplement and in the accompanying prospectus.

Other than in the United States, no action has been taken by us that would permit a public offering of the securities offered by this prospectus supplement and accompanying prospectus in any jurisdiction where action for that purpose is required. The securities offered by this prospectus supplement and accompanying prospectus may not be offered or sold, directly or indirectly, nor may this prospectus supplement and accompanying prospectus or any other offering material or advertisements in connection with the offer and sale of any such securities be distributed or published in any jurisdiction, except under circumstances that will result in compliance with the applicable rules and regulations of that jurisdiction. Persons into whose possession this prospectus supplement and accompanying prospectus come are advised to inform themselves about and to observe any restrictions relating to the offering and the distribution of this prospectus supplement and accompanying prospectus.

Unless the context otherwise indicates, references in this prospectus supplement to “we,” “our,” “us” and “company” refer to Forty Seven, Inc.

S-i

PROSPECTUS SUPPLEMENT SUMMARY

This summary highlights information contained in greater detail elsewhere in this prospectus supplement and the accompanying prospectus and the documents incorporated by reference. This summary is not complete and does not contain all of the information you should consider before investing in our common stock. You should read the entire prospectus supplement, the accompanying prospectus and the documents incorporated by reference carefully, especially the risks of investing in our common stock discussed under the heading “Risk Factors” in this prospectus supplement, the accompanying prospectus and our most recent Annual Report on Form10-K for the year ended December 31, 2018 and our most recent Quarterly Report on Form10-Q for the quarterly period ended March 31, 2019, each incorporated by reference herein, our financial statements and related notes in our most recent Annual Report and Quarterly Report and the section titled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” in our most recent Annual Report and Quarterly Report.

Forty Seven, Inc.

Overview

We are a clinical-stage immuno-oncology company focused on developing novel therapies to activate macrophages in the fight against cancer. We founded Forty Seven based on the insight that blocking CD47, a key signaling molecule that is overexpressed on cancer cells, renders tumors susceptible to macrophages. By harnessing macrophages, we believe that our lead product candidate, 5F9, can transform the treatment of cancer. 5F9 has demonstrated promising activity in multiple Phase 1b/2 clinical trials in which we have treated over 290 cancer patients with solid or hematologic tumors. In addition, we have two additional product candidates in preclinical development;FSI-189 an anti-SIRPa antibody andFSI-174 an anti-CKIT antibody.

We focus our efforts on targeting the CD47 pathway as a way to engage macrophages in fighting tumors. Macrophages function as first responders, swallowing foreign and abnormal cells, including cancer cells, and mobilizing other components of the immune system including T cells and antibodies. Cancer cells use CD47, a “don’t eat me” signal, in order to evade detection by the immune system and subsequent destruction by macrophages. Overexpression of CD47 is common to nearly all types of tumors and is also correlated with poor prognosis in multiple cancers including acute myelogenous leukemia, or AML, colorectal cancer, or CRC, gastric cancer, lung cancer,Non-Hodgkin’s lymphoma, or NHL, and ovarian cancer. Despite the central role of macrophages as cell-eating scavengers and first responders, the pharmaceutical industry is only beginning to bring this key group of cells into the fight against cancer.

Our company was founded by leading scientists at Stanford University who uncovered the fundamental role of CD47 in cancer evasion. Preclinical work performed in the laboratory of ourco-founder, Irving L. Weissman, at Stanford University and at Forty Seven demonstrated that:

• Blocking the CD47 “don’t eat me” signaling pathway leads to elimination of many types of tumors and increased survival;

• Boosting an “eat me” signal found on cancer cells using therapeutic antibodies results in a synergistic effect with blocking CD47;

• Macrophages digest cancer cells in a process called phagocytosis and present tumor-specific antigens that can activate T cells against the cancer, thus creating the potential for synergy with T cell checkpoint inhibitors; and

• Inducing an “eat me” signal on cancer cells using certain chemotherapies results in improved targeting of tumor cells which is synergistic with blocking CD47.

Our lead product candidate, 5F9, is a humanized IgG4 subclass monoclonal antibody against CD47 that is designed to interfere with recognition of CD47 by the SIRPa receptor on macrophages, thus blocking the “don’t

S-1

eat me” signal. The design of 5F9, combined with our proprietary dosing regimen, overcomes the toxicity limitations of previously tested anti-CD47 therapies developed by others. Across all study populations, 5F9 has been well tolerated with no maximum tolerated dose, or MTD, observed in any study despite dosing up to 45 mg/kg. The most common treatment-associated effects observed to date were the expected CD47-mechanism-based effects on red blood cells, which led to a temporary and reversible anemia. Other reported treatment-related adverse events include infusion reactions, headache, fatigue, chills, fever and nausea. The majority of these adverse events were mild to moderate in severity and were generally easily managed. To date, there are no approved therapies that target the CD47 checkpoint of the innate immune system.

The targeting of CD47 to make cancer cells susceptible to macrophages, a component of the innate immune system, is analogous to the approach that has been applied with checkpoint inhibitors and T cells, a component of the adaptive immune system. Since their introduction in 2011, T cell checkpoint inhibitors have become frontline therapies for certain cancers and we estimate that they generated over $17 billion in sales in 2018. Despite the success of T cell checkpoint inhibitors, these therapies have been shown to be effective only in a subset of tumors, highlighting the need for additional therapies. Similar to the way cancer cells overexpress programmed death-ligand 1, orPD-L1, to avoid attack by T cells, cancer cells overexpress CD47 as a way to avoid destruction by macrophages. We believe targeting CD47 represents a compelling and analogous approach.

Our Development Pipeline

We are conducting clinical trials using 5F9 as monotherapy and in combination with other approved cancer therapies. As summarized in the following figure, our clinical trials are investigating multiple types of CD47 combination therapies: 5F9 with chemotherapeutic agents, 5F9 with tumor targeting antibodies and 5F9 with T cell checkpoint inhibitors, in a wide variety of tumors, including both solid and hematological cancers. We have treated over 290 relapsed or refractory cancer patients with 5F9 both as a monotherapy and in combination with tumor targeting antibodies such as rituximab and cetuximab. While the primary goal of our trials has been to demonstrate safety, we have also observed early signs of clinical activity in multiple tumor types. These signs include patients with partial and complete responses, as well as patients with “stable disease.” We use standard clinical assessment criteria to evaluate the growth or reduction in existing tumor size, within set parameters, as well as growth of new tumors and metabolic activity. Broadly stated, “stable disease” indicates a growth or reduction in tumor size that is insufficient to meet the definitions of either progressive disease or partial or complete response. In contrast, patients with partial or complete responses have substantial reductions in tumor size.

S-2

5F9 Monotherapy

In our ongoing monotherapy trials, 5F9 treatment has demonstrated biological responses including a confirmed objective response and multiple cases of stable disease in Phase 1 patients with refractory AML, as of April 2018. In biologic responders, defined as a reduction in bone marrow blasts, we confirmed the presence of macrophages in tumor tissues and we observed that other components of the immune system, including T cells, had been recruited.

We are also investigating 5F9 as a monotherapy in ovarian cancer and other solid tumors. In a Phase 1 trial of 5F9, we observed confirmed partial responses in 2 out of 21 evaluable patients in a cohort with ovarian cancer receiving either 20 mg/kg or higher doses of 5F9, as of April 2018. Both were heavilypre-treated patients failing seven or more previous treatment regimens. One of these patients had a durable partial response of more than six months in duration.

We believe the signals from these monotherapy trials have been encouraging; however, we have determined that the limited responses in these late stage patients are not adequate for us to initiate a trial of 5F9 as a single agent aimed at supporting registration by the FDA and we are now focused on combination approaches that have a strong scientific rationale and that are backed by preclinical data.

5F9 in Combination with Tumor Targeting Antibodies

We are also pursuing multiple trials of 5F9 in combination with tumor targeting antibodies in order to test the synergistic potency of these combinations. We believe that we can enhance the effect of 5F9 on cancer by using tumor targeting antibodies that bind to cancer cells and present an “eat me” signal to macrophages. Hence, we are combining 5F9 with tumor targeting antibodies such as rituximab and cetuximab. Based on our preclinical research and on publications by academic groups, we believe that this combination of an “eat me” signal by these

S-3

antibodies and the blocking of a “don’t eat me” signal by 5F9 could be highly effective. We are conducting a Phase 1b/2 combination trial using 5F9 and rituximab in patients with relapsed and refractory NHL. As of April 2018, 30 patients with refractory NHL have been evaluated in Phase 1b/2 and 14 (47%) have had an objective response during the dose finding study of 5F9 in combination with rituximab. In 10 (33%) of these patients, we observed a complete response, an uncommon therapeutic finding for such a heavilypre-treated population. In November 2018, the Phase 1b NHL findings were published in the New England Journal of Medicine. Based on our application summarizing the early NHL trial data, the FDA granted Fast Track designations to 5F9 for the treatment of both relapsed and/or refractory diffuse large B cell lymphoma, or DLBCL, and relapsed and/or refractory follicular lymphoma, or FL, in April 2018. Having obtained Fast Track status, we held an end of Phase 1 meeting with the FDA in July 2018 to further discuss our NHL trials. Interim results from this trial are expected inmid-2019. In May 2019, we announced a clinical trial collaboration with Acerta Pharma, AstraZeneca’s hematology research and development center of excellence to evaluate the triple combination of 5F9 and rituximab with CALQUENCE (acalabrutinib), an inhibitor of Bruton Tyrosine Kinase, in patients with DLBCL. We will supply 5F9 and Acerta will conduct the study. We are also conducting a Phase 1b/2 combination clinical trial using 5F9 and cetuximab in patients with CRC. Results from this trial are expected in the second half of 2019.

5F9 in Combination with Chemotherapeutic Agents

We are also exploring a combination of azacitidine, a chemotherapeutic agent, with 5F9 in patients with untreated AML and myelodysplastic syndromes, or MDS. We have shown in preclinical studies that azacitidine induces “eat me” signals on AML cells which leads to enhanced phagocytosis when combined with 5F9. These results were presented at the 2018 American Society of Hematology meeting. We are conducting a Phase 1b trial of 5F9 with azacitidine in untreated AML and MDS patients to evaluate the safety and efficacy of this combination therapy. Please see “—Recent Developments” below.

5F9 Combinations with Checkpoint Inhibitors

We believe there is a strong rationale to combine 5F9 and T cell checkpoint inhibitors and we plan to initiate combination clinical trials in both solid and hematological tumors. 5F9 induces a potent anti-tumor T cell response by enabling macrophages to ingest cancer cells and present antigens derived from these cancer cells to T cells. Thus, we believe the combination of a T cell checkpoint inhibitor with 5F9 is likely to further enhance an anti-tumor T cell response and to further mobilize both the innate and adaptive immune systems to eliminate cancer.

In early 2018, we announced clinical trial collaboration and supply agreements with two pharmaceutical companies to combine 5F9 withPD-L1 checkpoint inhibitors, while retaining full economic rights to our products. Pursuant to these agreements, we are conducting clinical trials with Merck KGaA on the combination of 5F9 with BAVENCIO (avelumab) in ovarian cancer patients; and with Genentech, Inc., a member of the Roche Group, on the combination of 5F9 and TECENTRIQ (atezolizumab) in patients with bladder cancer and in patients with AML. In April 2019, we announced an extension of our clinical trial collaboration agreement with Genentech to also evaluate 5F9 in combination with rituximab and TECENTRIQ in patients with DLBCL. We will supply 5F9, and Merck KGaA and Genentech will supply their respective drug products for these trials.

Recent Developments

5F9 for the Treatment of MDS

Clinical Trial Update

In June 2019, we announced results from our Phase 1b trial designed to evaluate 5F9 as a monotherapy in patients with relapsed or refractory, or r/r, MDS or AML, and 5F9 in combination with azacitidine in higher-risk MDS patients

S-4

and untreated, induction chemotherapy-ineligible AML patients. All patients received a 1 mg/kg priming dose of 5F9, coupled with intrapatient dose escalation, to mitigateon-target anemia. Patients in the combination cohort were then treated with full doses of azacitidine and a 5F9 maintenance dose of 30 mg/kg once weekly.

As of the data cutoff of May 10, 2019, 46 patients had been treated in the Phase 1b portion of the trial, including 10 r/r MDS or AML patients who received monotherapy 5F9, and 36 untreated higher-risk MDS patients or untreated AML patients ineligible for induction chemotherapy, who received 5F9 in combination with azacitidine.

Safety data are available for 5F9 in 10 patients treated with monotherapy and for 36 patients treated in combination with azacitidine. In both groups, 5F9 treatments were well-tolerated, and in the combination there was no evidence of increased toxicities compared to azacitidine alone. The number and type of adverse events, or AEs, observed were generally consistent with what has been previously seen with 5F9, and no significant cytopenias or autoimmune-related AEs were observed in patients treated with monotherapy 5F9. Overall, the most commonly reported treatment-related AEs were expected CD47-mechanism-based effects on red blood cells, which led to a temporary and reversible anemia with the initial dose. After the first few doses many patients in the combination cohort experienced a hemoglobin improvement over the course of their treatment with a decrease in transfusions. Importantly, no treatment-related infections were observed, and only one patient out of 36 treated with the combination experienced neutropenic fever (3%). No deaths were observed in the first 60 days on combination treatment. Only one patient out of 46 (2%) discontinued treatment due to a treatment-related AE.

Thirty-five patients were evaluable for response assessment, including 25 patients with untreated higher-risk MDS or AML who were treated with 5F9 and azacitidine (11 patients with higher-risk MDS and 14 patients with untreated AML) and 10 patients with r/r MDS or AML who were treated with monotherapy 5F9.

• In higher-risk MDS, the overall response rate, or ORR, for the combination was 100%, with six patients (55%) achieving a complete response, or CR, four patients (36%) achieving a marrow CR and one patient (9%) achieving hematologic improvement.

• In untreated AML, the ORR for the combination was 64%, with five patients (36%) achieving a CR, two patients (14%) achieving a complete response with incomplete blood count recovery, or CRi, and two patients (14%) achieving a morphologic leukemia-free state, or MLFS. Additionally, five patients (36%) achieved stable disease, or SD.

• In r/r MDS or AML treated with monotherapy 5F9, the ORR was 10%, consisting of one patient who achieved a MLFS. Additionally, seven patients (70%) achieved SD.

• The median time to response among MDS and AML patients treated with the combination was 1.9 months.

• Six patients (30% of responders) receiving the combination who had an objective response have experienced deepening responses over time resulting in complete remissions. Five patients (25% of responders) have also successfully received allogeneic stem cell transplants.

• Historical response rates for single-agent azacitidine show CR rates of approximately15-20% in higher-risk MDS and untreated AML patients, with initial responses generally occurring after4-6 months in most patients who respond.

Based on the favorable safety profile and encouraging clinical activity observed in this Phase 1b clinical trialto-date, expansion cohorts have been initiated in patients with both higher-risk MDS and untreated AML with 5F9 in combination with azacitidine.

S-5

Registration Strategy Update

Based on feedback from a Type B meeting with the FDA we believe asingle-arm trial design may be sufficient to support the approval of 5F9 in combination with azacitidine for the treatment of naïve (1st line) intermediate to very high risk MDS patients. We plan to finalize the key parameters with the FDA on the trial elements through a Special Protocol Assessment.

The primary endpoint of this single-arm study is expected to be a durable objective response consisting of both complete and partial responses. We believe approximately 91 patients with six months of efficacy data and 12 months of safety data may be sufficient for submission of a Biologic License Application, or BLA. We further believe there is a potential accelerated approval pathway utilizing thissingle-arm trial design.

We also plan to expand our current Phase 1b/2 trial, with weekly 30 mg/kg dosing, to accrue a total of 91 patients, and we anticipate completing enrollment of this trial in the third quarter of 2020. We plan to use the 12 months of safety data from this trial as part of our BLA submission. In addition, in the first quarter of 2020, we plan to initiate a second trial of 5F9 plus azacitidine in untreated intermediate to very high risk MDS patients with dosing every two weeks. We plan to enroll approximately 91 patients in this trial, which is expected to allow us to explore a more convenient dosing regimen. At the completion of both trials, we intend to evaluate the data and determine which dosing regimen to submit as part of our BLA submission. We currently anticipate submitting our BLA in the fourth quarter of 2021.

5F9 for the Treatment of NHL

Clinical Trial Update

In June 2019, we also announced updated data from our ongoing Phase 1b/2 clinical trial evaluating 5F9 in combination with rituximab for the treatment of relapsed/refractorynon-Hodgkin’s lymphoma, or r/r NHL, including DLBCL and indolent lymphoma. These data showed clinical benefit across a range of patient populations — including patients who are heavilypre-treated, ineligible forCAR-T therapy or suffering from primary refractory disease — as well as durable responses in both DLBCL and FL.

The design of our Phase 1b/2 clinical trial allowed us to continue to explore the clinical benefit of 5F9 in combination with rituximab in patients with DLBCL and indolent lymphoma, while also expanding into a subset of older, sicker DLBCL patients who have been deemed ineligible forCAR-T therapy, a newly-defined population, which has never before been evaluated in clinical trials, and for whom there are few, if any, effective treatment options available.

Our Phase 1b/2 NHL trial is designed to evaluate 5F9 in combination with rituximab in patients with r/rB-cell NHL, who have failedstandard-of-care therapies. All patients received a 1 mg/kg priming dose of 5F9 to mitigateon-target anemia. Patients in the Phase 1b portion of the trial were treated with 5F9 maintenance doses of 10 to 45 mg/kg, and patients in the Phase 2 portion of the trial were treated with 5F9 doses of either 30 or 45 mg/kg. All patients were also administered full doses of rituximab.

As of the data cutoff of May 2019, 115 patients had been treated in the Phase 1b/2 trial, including 70 patients with DLBCL, 41 patients with follicular lymphoma (FL) and four patients with marginal zone lymphoma, or MZL. The median number of prior therapies across all patients was three (range one to 10), and 85% of all patients were refractory to a prior rituximab-containing regimen, with 59% of DLBCL patients having primary refractory disease. Additionally, 42 of the 47 DLBCL patients enrolled in the Phase 2 portion of the trial were ineligible forCAR-T therapy (89%).

In this trial, 5F9 was well tolerated in combination with rituximab. The number and type of AEs were consistent with prior clinical experience. Most AEs were Grade 1 or Grade 2, and the most commonly-reported

S-6

AEs were expected CD47-mechanism-based effects on red blood cells, which led to a temporary and reversible anemia, and infusion-site reactions. No autoimmune-related AEs were observed, nor were any significant late safety signals observed in patients treated with 5F9 for up to 24 months. No maximum tolerated dose was reached with up to 45 mg/kg of 5F9 dosing. Eight out of 115 patients discontinued treatment due to an AE (7%).

Ninety-seven patients were evaluable for response assessment, including 21 relapsed/refractory DLBCL patients who were treated in the Phase 1b portion of the study, thirty-eight DLBCL patients who were treated in the Phase 2 portion of the study and 38 indolent lymphoma patients (35 patients with FL and three patients with MZL).

DLBCL

Best Overall Response	Phase 1b N = 21 (%)	Phase 2 N = 38 (%)	³ 3 Prior Lines of Therapy N = 39 (%)
Study Patient Population	Primary refractory disease or relapsed/refractory to ³ 2 prior lines of therapy	Primary refractory disease or relapsed/refractory to³ 2 prior lines of therapy and ineligible forCAR-T therapy	Subgroup analysis of combined Phase 1b and Phase 2 Data
ORR	10 (48)%	11 (29)%	15 (38)%
CR	7 (33)%	2 (5)%	7 (18)%
PR	3 (14)%	9 (24)%	8 (20)%
SD	4 (19)%	3 (8)%	4 (10)%

Among patients treated in the Phase 1b portion of the trial, the median duration of response has not been reached, with a medianfollow-up of over 13.8 months. This includes one patient who has remained in a durable CR for more than 24 months.

Indolent Lymphoma

Best Overall Response	Phase 1b + 2 FL N = 35; MZL N = 3 (%)
Study Patient Population	Relapsed/refractory to³ 2 prior lines of therapy
ORR	23 (61)%
CR	9 (24)%
PR	14 (37)%
SD	9 (24)%

Among patients treated in the Phase 1b portion of the trial, the median duration of response has not been reached with a medianfollow-up of over 21 months. This includes the patient who has remained in a durable CR for more than 28 months.

Additionally, 5F9 tumor penetrance was evaluated at 30 and 45 mg/kg as a key pharmacodynamic endpoint. Data show that the 30 mg/kg maintenance dose of 5F9 saturated the tumor microenvironment similarly to 45 mg/ kg, with similar efficacy. As a result, a 30 mg/kg maintenance dose of 5F9 was selected as the recommended dose for use in future clinical studies.

We also intend to evaluate opportunities to advance 5F9 in combination with rituximab for patients with indolent lymphoma.

S-7

In February 2019 we dosed our first patient in an expansion cohort of our 5F9003 NHL Phase 1 trial with a combination of 5F9 plus rituximab with gemcitabine and oxaliplatin. We anticipate enrolling up to 26 patients into this cohort. We anticipate sharing data from this cohort in the second half of 2020.

Registration Strategy Update

Based on feedback from a Type C meeting with the FDA in May 2019 we believe a single-arm trial design may be sufficient to support the approval of 5F9 plus rituximab in heavily pretreated relapsed or refractory DLBCL patients, includingCAR-T ineligible patients.

We plan to initiate a registrational trial of 5F9 plus rituximab in DLBCL in the first quarter of 2020. We anticipate this trial will enroll approximately 100 patients with advanced forms of DLBCL using refined patient eligibility criteria based on our Phase 1 and 2 trials. We anticipate these will be patients that have had more than two or more lines of therapy, and may include those ineligible forCAR-T therapy due to advanced age, significant co-morbidities or the diagnosis of rapidly progressive disease. The dosing regimen for the trial is anticipated to be 30 mg/kg every two weeks. The primary endpoints of the study are expected to be objective response rate, including both complete and partial responses, and durability of response. Because theCAR-T ineligible patient population has not been well characterized before, the FDA has requested informational data to better characterize and defineCAR-T ineligibility. The anticipated efficacy follow up in this trial will be six months. We further believe there is a potential accelerated approval pathway utilizing this single-arm trial design.

Key Clinical and Regulatory Events Expected in 2019 and 2020

We expect to provide updates from our ongoing clinical trials at major medical meetings throughout the year. Notably, we expect to provide an interim efficacy readout from our DLBCL trial of 5F9 plus rituximab in the fourth quarter of 2020 and expanded efficacy and durability from our MDS trial of 5F9 plus azacitidine in the second half of 2019 and 2020. Our expected key clinical and regulatory events are shown in the table below.

S-8

Acerta Pharma Collaboration

In May 2019, we entered into a collaboration with Acerta Pharma, AstraZeneca’s hematology research and development center of excellence, to evaluate 5F9 in combination with rituximab plus AstraZeneca/Acerta Pharma’s Bruton’s tyrosine kinase, or BTK, inhibitor, acalabrutinib (CALQUENCE), in patients with DLBCL. This approach aims to optimize the treatment of DLBCL patients by inhibiting BTK. InB-cells, BTK signaling results in the activation of pathways associated withB-cell proliferation, trafficking, chemotaxis and adhesion, and BTK inhibition is a proven strategy for inducing the regression ofB-cell lymphomas.

Genentech Collaboration

In April 2019, we extended our existing collaboration with Genentech, a member of the Roche Group, to include a third clinical trial evaluating 5F9 in combination with rituximab plus Genentech’santi-PD-L1 antibody, atezolizumab (TECENTRIQ), in patients with DLBCL. This approach aims to optimize the treatment of DLBCL patients whose tumors are associated with high levels of macrophages expressingPD-L1.

In June 2019, a clinical trial collaboration with Genentech, to explore 5F9 plus atezolizumab for the treatment of bladder cancer, dosed the first patient in a Phase 1 clinical trial.

Ono Pharmaceutical License and Collaboration Agreement

On July 10, 2019, we entered into an exclusive license and collaboration agreement with Ono Pharmaceutical Co., Ltd., or Ono. Under the agreement, we granted Ono an exclusive license to develop, manufacture and commercialize 5F9, our monoclonal antibody against CD47, as well as other anti-CD47 antibodies controlled by us in Japan, South Korea, Taiwan and the ASEAN countries, or the Ono Territory. We retain all rights to 5F9 and other licensed antibodies outside of the Ono Territory.

Under the agreement, the parties will collaborate on the development, manufacturing and commercialization of 5F9 and other licensed antibodies. Each party will be responsible for conducting development and commercialization of licensed antibodies in its respective territory at its own cost. Further, each party will have the right to participate, at its own cost, in global clinical studies of 5F9 and other licensed antibodies conducted by the other party. We will initially be responsible for supplying 5F9 and other licensed antibodies to Ono for development and commercialization within the Ono Territory at Ono’s cost. Ono has the right to elect that such manufacturing activities be transferred to Ono. During the term of the agreement, neither party may manufacture or commercialize any competing products in the Ono Territory.

We will receive aone-time upfront payment of approximately $15.8 million from Ono and will be eligible to receive up to an additional approximately $104 million at current exchange rates if specified future development and commercial milestones are achieved by Ono. We are also eligible to receive tiered percentage royalties spanning from themid-teens to thelow-twenties on future net sales of 5F9 and other licensed antibodies in the Ono Territory, subject to certain offsets. Ono’s obligation to pay royalties expires, on aproduct-by-product andcountry-by-country basis, on the later of (1) the expiration of the first regulatory exclusivity for such product in such country, (2) the expiration of the last to expire patent controlled by us that covers the composition of matter of a licensed antibody in such product in such country, or (3) the tenth anniversary of the first commercial sale of such product in such country.

The agreement will remain in effect until the expiration of all of Ono’s royalty obligations, after which Ono’s license shall be fullypaid-up. Ono may terminate the agreement on acountry-by-country basis for convenience upon 90 days’ prior written notice to us prior to the first commercial sale of the first licensed product in the Ono Territory, or 180 days’ prior written notice after such first sale. Either party may also

S-9

terminate the agreement for the other party’s uncured material breach or insolvency, subject to specified notice and cure periods. In the event of any early termination, all rights in 5F9 and other licensed antibodies will revert to us, subject to certain royalties due to Ono in the case of Ono’s termination for our breach or insolvency.

Corporate Information

We were incorporated in Delaware in 2014 as CD47 Sciences, Inc. Our principal executive offices are located at 1490 O’Brien Drive, Suite A, Menlo Park, California 94025, and our telephone number is (650)352-4150.

Our website address is www.fortyseveninc.com. Information contained on, or that can be accessed through, our website is not incorporated by reference into this prospectus supplement and the accompanying prospectus, and you should not consider information on our website to be part of this prospectus supplement or the accompanying prospectus.

“Forty Seven,” the Forty Seven logo and other trademarks or service marks of Forty Seven appearing in this prospectus supplement and the accompanying prospectus are our property. This prospectus supplement and the accompanying prospectus contain additional trade names, trademarks, and service marks of other companies, which are the property of their respective owners. We do not intend our use or display of other companies’ trade names, trademarks or service marks to imply a relationship with, or endorsement or sponsorship of us by, these other companies.

S-10

THE OFFERING

Common stock offered by us.	9,375,000 shares
Option to purchase additional shares	1,406,250 shares
Common stock to be outstanding after this offering	40,673,599 shares
Use of proceeds	We estimate that the net proceeds from this offering will be approximately $70.1 million (or approximately $80.7 million if the underwriters’ option to purchase additional shares is exercised in full) based on the public offering price of $8.00 per share, after deducting underwriting discounts and commissions and estimated offering expenses payable by us. We intend to use the net proceeds from this offering, together with our cash, cash equivalents and short-term investments, to conduct our clinical trials, to fund continued research and development of 5F9 in several applications, to fund other research and development activities, and for working capital and other general corporate purposes. We may also use a portion of the net proceeds to license intellectual property or to make acquisitions or investments, although we have no commitments or agreements to enter into such licenses, acquisitions or investments. See the section titled “Use of Proceeds” for additional information.
Risk factors	See “Risk Factors” section of this prospectus supplement, as well as those risk factors that are incorporated by reference in this prospectus supplement and the accompanying prospectus for a discussion of factors you should carefully consider before deciding to invest in our common stock.
Nasdaq Global Select Market trading symbol	“FTSV”

Jeffrey W. Bird, a member of our board of directors, and Bluebird Ventures, his affiliate, as well as entities affiliated with Lightspeed Venture Partners, an existing stockholder, and the Leukemia & Lymphoma Society, Inc., a strategic collaborator and an existing stockholder that had submitted indications of interest, have agreed to purchase an aggregate of 1,750,000 shares of our common stock in this offering at the public offering price.

The number of shares of common stock that will be outstanding after this offering is based on 31,298,599 shares of common stock outstanding as of March 31, 2019, and excludes:

• 3,223,178 shares of common stock issuable upon the exercise of outstanding stock options as of March 31, 2019, with a weighted-average exercise price of $6.72 per share;

• 2,861,930 shares of common stock reserved for future issuance under our 2018 Equity Incentive Plan as of March 31, 2019, as well as any automatic increases in the number of shares of common stock reserved for future issuance under this plan; and

S-11

• 716,136 shares of common stock reserved for issuance under our 2018 Employee Stock Purchase Plan as of March 31, 2019, as well as any automatic increases in the number of shares of common stock reserved for future issuance under this plan.

In addition, unless we specifically state otherwise, all information in this prospectus supplement assumes:

• no exercise of the outstanding options described above; and

• no exercise of the underwriters’ option to purchase additional shares.

S-12

SUMMARY FINANCIAL DATA

We have derived the summary statement of operations data for the years ended December 31, 2017 and 2018 from our audited financial statements incorporated by reference into this prospectus. We derived the summary statement of operations data for the three months ended March 31, 2018 and 2019 and the summary balance sheet data as of March 31, 2019 from our unaudited interim condensed financial statements and related notes incorporated by reference into this prospectus. Our unaudited interim condensed financial statements were prepared on the same basis as our audited financial statements and, in our opinion, reflect all adjustments, consisting only of normal recurring adjustments that are necessary for the fair statement of our unaudited interim condensed financial statements. You should read the following summary financial data in conjunction with “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and our financial statements and related notes incorporated by reference into this prospectus. Our historical results are not necessarily indicative of the results to be expected for any other period in the future, and our interim results are not necessarily indicative of the results to be expected for the full year or any other period.

	Year Ended December 31,						Three Months Ended March 31,
	2017			2018			2018			2019
	(In thousands, except share and per share data)
Statement of Operations Data:
Operating expenses:
Research and development	$	37,174		$	56,673		$	11,153		$	19,126
General and administrative		8,130			15,432			3,843			4,584
Total operating expenses		45,304			72,105			14,996			23,710
Loss from operations		(45,304	)		(72,105	)		(14,996	)		(23,710	)
Interest and other income, net		406			1,735			221			694
Net loss	$	(44,898	)	$	(70,370	)	$	(14,775	)	$	(23,016	)
Net loss per share, basic and diluted(1)	$	(6.94	)	$	(3.75	)	$	(2.24	)	$	(0.74	)
Shares used in computing net loss per share, basic and diluted(1)		6,468,634			18,768,868			6,600,407			31,166,184

(1) See the statements of operations and Note 9 to our audited financial statements for the year ended December 31, 2018, and Note 8 to our unaudited interim condensed financial statements for the three months ended March 31, 2019, for further details on the calculation of net loss per share, basic and diluted.

	March 31, 2019
	(In thousands)
Balance Sheet Data:
Cash, cash equivalents and short-term investments	$	113,589
Total assets		130,311
Working capital		109,408
Total liabilities		17,412
Accumulated deficit		(162,785	)
Total stockholders’ equity		112,899

S-13

Financial Update

On a preliminary unaudited basis, our cash, cash equivalents and short-term investments as of June 30, 2019 was approximately $99.0 million. This balance of cash, cash equivalents and short-term investments is our preliminary estimate based on currently available information. It does not present all necessary information for an understanding of our financial condition as of June 30, 2019 or our results of operations for the three and six months ended June 30, 2019. As we completeour quarter-end financial close process and finalize our second quarter 2019 unaudited financial statements, we will be required to make significant judgments in a number of areas that may result in the preliminary balance of cash, cash equivalents and short-term investments provided herein being different than the final financial information. This preliminary balance has been prepared by and is the responsibility of our management. Our independent registered public accounting firm has not audited, reviewed or performed any procedures with respect to this preliminary balance or the accounting treatment thereof and does not express an opinion or any other form of assurance with respect thereto. We expect to complete our unaudited financial statements for the quarter ended June 30, 2019 subsequent to the completion of this offering. It is possible that we or our independent registered public accounting firm may identify items that require us to make adjustments to the preliminary balance of cash, cash equivalents and short-term investments set forth above and those changes could be material. Accordingly, undue reliance should not be placed on this preliminary balance. The preliminary balance is not necessarily indicative of any future period and should be read together with the sections titled “Risk Factors” and “Special Note Regarding Forward-Looking Statements” in this prospectus supplement, the accompanying prospectus and our financial statements, related notes and other financial information incorporated by reference herein.

S-14

RISK FACTORS

Investing in our common stock involves a high degree of risk. You should carefully consider the risks and uncertainties described below, together with the “Risk Factors” section of our Annual Report on Form10-K for the year ended December 31, 2018 and our Quarterly Report on Form10-Q for the three months ended March 31, 2019, each incorporated by reference herein, as well as all of the other information contained in this prospectus supplement, the accompanying prospectus or incorporated by reference herein, including our financial statements and the related notes, before deciding to invest in our common stock. If any of the following risks actually occur, it could harm our business, prospects, operating results and financial condition. Unless otherwise indicated, references to our business being harmed in these risk factors will include harm to our business, reputation, financial condition, results of operations, revenue and future prospects. In such event, the trading price of our common stock could decline and you might lose all or part of your investment.

If you purchase shares of our common stock in this offering, you will suffer immediate dilution of your investment.

The public offering price of our common stock in this offering is substantially higher than the as adjusted net tangible book value per share of our common stock. Therefore, if you purchase shares of our common stock in this offering, you will pay a price per share that substantially exceeds our as adjusted net tangible book value per share after this offering. Based on the public offering price of $8.00 per share, you will experience immediate dilution of $3.50 per share, representing the difference between our as adjusted net tangible book value per share after this offering and the public offering price. In addition, to the extent outstanding stock options are exercised, there will be further dilution to investors in this offering. In addition, if the underwriters exercise their option to purchase additional shares in full, or if we issued additional equity securities, you will experience additional dilution. See “Dilution” for a more detailed description of the dilution to investors in the offering.

We will have broad discretion in the use of the net proceeds from this offering and may not use them effectively.

Our management will have broad discretion in the application of the net proceeds from this offering and could spend the proceeds in ways that do not improve our results of operations or enhance the value of our common stock. We intend to use the net proceeds from this offering, together with our cash, cash equivalents and short-term investments, to conduct our clinical trials, to fund continued research and development of 5F9 in several applications, to fund other research and development activities, and for working capital and other general corporate purposes. We may also use a portion of the net proceeds to license intellectual property or make acquisitions or investments, although we have no commitments or agreements to enter into such licenses, acquisitions or investments. See “Use of Proceeds.” The failure by our management to apply these funds effectively could result in financial losses that could have an adverse effect on our business, cause the price of our common stock to decline and delay the development of our product candidates. Pending their use, we may invest the net proceeds from this offering in a manner that does not produce income or that loses value.

S-15

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein contain forward-looking statements about us and our industry within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended, that involve substantial risks and uncertainties. All statements other than statements of historical facts contained in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference, including statements regarding our future financial condition, results of operations, business strategy and plans, and objectives of management for future operations, as well as statements regarding industry trends, are forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intend,” “may,” “plan,” “potentially,” “predict,” “should,” “will,” “project,” “target,” “contemplate” or the negative of these terms or other similar expressions.

We have based these forward-looking statements largely on our current expectations and projections about future events and trends that we believe may affect our financial condition, results of operations, business strategy and financial needs. These forward-looking statements are subject to a number of risks, uncertainties, factors and assumptions described under the section titled “Risk Factors” and elsewhere in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference, regarding, among other things:

• the success, cost and timing of our product development activities and clinical trials;

• our expectations about the timing of regulatory submissions and achieving regulatory approvals;

• our ability to obtain funding for our operations, including funding necessary to complete further development and commercialization of our product candidates;

• the commercialization of our product candidates, if approved;

• our plans to research, develop and commercialize our product candidates;

• our ability to maintain, expand, protect and enforce our intellectual property portfolio;

• our ability to operate our business without infringing, misappropriating or otherwise violating the intellectual property rights of third parties;

• our ability to attract collaborators with development, regulatory and commercialization expertise;

• our expectations with respect to our collaboration agreements;

• our expectations regarding our ability to obtain and maintain intellectual property protection for our product candidate;

• our ability to reach agreements and work with third parties in connection with the development and commercialization of our product candidates;

• the size and growth potential of the markets for our product candidates, and our ability to serve those markets;

• the rate and degree of market acceptance of our product candidates;

• regulatory developments in the United States and foreign countries;

• our ability to contract with third-party suppliers and manufacturers and their ability to perform adequately;

• the success of competing therapies that are or may become available;

S-16

• our ability to attract and retain key scientific or management personnel;

• the accuracy of our estimates regarding expenses, future revenue, capital requirements and needs for additional financing;

• our expectations regarding the period during which we qualify as an emerging growth company under the JOBS Act;

• our use of the proceeds from this offering; and

• our ability to maintain proper and effective internal controls.

These risks are not exhaustive. Other sections of this prospectus supplement and the accompanying prospectus may include additional factors that could harm our business and financial performance. New risk factors may emerge from time to time and it is not possible for our management to predict all risk factors, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in, or implied by, any forward-looking statements.

You should not rely upon forward-looking statements as predictions of future events. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements. Except as required by law, we undertake no obligation to update publicly any forward-looking statements for any reason after the date of this prospectus supplement or to conform these statements to actual results or to changes in our expectations.

In addition, statements that “we believe” and similar statements reflect our beliefs and opinions on the relevant subject. These statements are based upon information available to us as of the date of this prospectus supplement, and while we believe such information forms a reasonable basis for such statements, such information may be limited or incomplete, and our statements should not be read to indicate that we have conducted an exhaustive inquiry into, or review of, all potentially available relevant information. These statements are inherently uncertain and investors are cautioned not to unduly rely upon these statements.

You should read this prospectus supplement, the accompanying prospectus and the documents we have filed with the Securities and Exchange Commission, or SEC, that are incorporated by reference with the understanding that our actual future results, levels of activity, performance and achievements may be different from what we expect. We qualify all of our forward-looking statements by these cautionary statements.

S-17

INDUSTRY AND MARKET DATA

This prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein contain estimates, projections and other information concerning our industry and our business, including estimated market size, projected growth rates and the incidence of certain medical conditions. Unless otherwise expressly stated, we obtained this industry, business, market, medical and other information from reports, research surveys, studies and similar data prepared by third parties, industry, medical and general publications, government data and similar sources. In some cases, we do not expressly refer to the sources from which this information is derived. In that regard, when we refer to one or more sources of this type of information in any paragraph, you should assume that other information of this type appearing in the same paragraph is derived from the same sources, unless otherwise expressly stated or the context otherwise requires.

This industry, business, market, medical and other information involves a number of assumptions and limitations, and you are cautioned not to give undue weight to such estimates. We have not independently verified any third-party information and cannot assure you of its accuracy or completeness. Although we are responsible for all of the disclosure contained in this prospectus supplement and the accompanying prospectus and we believe the market position, market opportunity, market size and medical information included in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein is reliable, such information is inherently imprecise. In addition, projections, assumptions and estimates of our future performance and the future performance of the industry in which we operate is necessarily subject to a high degree of uncertainty and risk due to a variety of factors, including those described in the section titled “Risk Factors” and elsewhere in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein. These and other factors could cause results to differ materially from those expressed in the estimates made by the independent parties and by us.

S-18

USE OF PROCEEDS

We estimate that the net proceeds to us from the sale of 9,375,000 shares of common stock in this offering will be approximately $70.1 million (or approximately $80.7 million if the underwriters exercise their option to purchase 1,406,250 additional shares in full) at the public offering price of $8.00 per share, after deducting underwriting discounts and commissions and estimated offering expenses payable by us.

We currently expect to use our net proceeds from this offering, together with our cash, cash equivalents and short-term investments, as follows:

• approximately $50.0 to $55.0 million to further the clinical development of 5F9 through interim data readouts of registrational studies we plan to initiate in DLBCL and MDS based on our evaluation of clinical data and recent regulatory feedback;

• approximately $3.0 to $7.0 million to further the clinical development of 5F9 through completion or interim data readouts of our existing Phase 1/2 clinical trials, including our monotherapy and combination trials with tumor-targeting antibodies, T cell checkpoint inhibitors and chemotherapy;

• approximately $10.0 to $12.0 million to further the development of our anti-SIRPa antibody product candidate,FSI-189, and our anti-cKIT antibody product candidate,FSI-174, including initiation of Phase 1 clinical studies for both programs; and

• the remaining proceeds for research and drug discovery activities related to additional product candidates, working capital and other general corporate purposes.

We expect that our operating expenses will continue to increase in 2019 and beyond as we advance our development pipeline and accelerate our efforts to conduct clinical trials and obtain regulatory approval for our lead product candidate 5F9 in one or more indications. Based on our current operating plans and assumptions, we believe our existing cash, cash equivalents and short-term investments and upfront payment from Ono, together with the expected net proceeds from this offering, will enable us to meet our financial needs into the first quarter of 2021. However, we will require additional funds to complete our currently planned clinical trials and development programs.

Due to the uncertainties inherent in the product development process, it is difficult to estimate with certainty the exact amounts of the net proceeds from this offering that may be used for the above purposes. Our management will have broad discretion over the use of the net proceeds from this offering. The amounts and timing of our expenditures will depend upon numerous factors including the results of our research and development efforts, the timing and success of preclinical studies and any ongoing clinical trials or clinical trials we may commence in the future, the timing of regulatory submissions and the amount of cash obtained through future collaborations, if any. Following this offering, we will require additional funding in order to complete clinical development and commercialize our lead product candidate, 5F9, and complete the clinical development of any additional product candidates.

In addition, we believe opportunities may exist from time to time to expand our current business through acquisitions orin-licenses of, or investments in, complementary companies, medicines or technologies. While we have no current agreements, commitments or understandings for any specific acquisitions,in-licenses or investments at this time, we may use a portion of the net proceeds for these purposes.

Pending the use of the proceeds from this offering as described above, we intend to invest the net proceeds in interest-bearing investment-grade securities or government securities.

S-19

DIVIDEND POLICY

We have never declared or paid cash dividends on our capital stock. We currently intend to retain all available funds and any future earnings, if any, to support operations and to finance the growth and development of our business. We do not intend to declare or pay cash dividends on common stock in the foreseeable future.

S-20

CAPITALIZATION

The following table sets forth our cash, cash equivalents and short-term investments and our capitalization as of March 31, 2019, on:

• an actual basis; and

• an as adjusted basis to reflect the sale of 9,375,000 shares of common stock in this offering, at the public offering price of $8.00 per share, after deducting underwriting discounts and commissions and estimated offering expenses payable by us.

You should read this information together with our financial statements and related notes and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” in our Annual Report on Form10-K for the year ended December 31, 2018 and our Quarterly Report on Form10-Q for the three months ended March 31, 2019 and other documents incorporated by reference in this prospectus supplement and the accompanying prospectus.

	As of March 31, 2019
	Actual			As Adjusted
	(In thousands, except share and per share data)
Cash, cash equivalents and short-term investments	$	113,589		$	183,689
Stockholders’ equity:
Preferred stock, $0.0001 par value per share: 10,000,000 shares authorized, and no shares issued and outstanding, actual and as adjusted	$	—		$	—
Common stock, $0.0001 par value per share: 200,000,000 shares authorized, 31,298,599 shares issued and outstanding, actual; 40,673,599 shares issued and outstanding, as adjusted		3			4
Additionalpaid-in capital		275,663			345,762
Accumulated other comprehensive loss		18			18
Accumulated deficit		(162,785	)		(162,785	)
Total stockholders’ equity		112,899			182,999
Total capitalization	$	112,899		$	182,999

The outstanding share information in the table above excludes, as of March 31, 2019, the following shares:

• 3,223,178 shares of common stock issuable upon the exercise of outstanding stock options as of March 31, 2019, with a weighted-average exercise price of $6.72 per share;

• 2,861,930 shares of common stock reserved for future issuance under our 2018 Equity Incentive Plan as of March 31, 2019, as well as any automatic increases in the number of shares of common stock reserved for future issuance under this plan; and

• 716,136 shares of common stock reserved for issuance under our 2018 Employee Stock Purchase Plan as of March 31, 2019, as well as any automatic increases in the number of shares of common stock reserved for future issuance under this plan.

S-21

DILUTION

If you invest in our common stock in this offering, your ownership interest will be diluted immediately to the extent of the difference between the public offering price per share of our common stock and the adjusted net tangible book value per share of our common stock immediately after the closing of this offering.

Our net tangible book value of our common stock as of March 31, 2019 was $112.9 million, or $3.61 per share, based on the total number of shares of our common stock outstanding as of March 31, 2019. Net tangible book value per share represents our total tangible assets less our total liabilities, divided by the number of outstanding shares of common stock.

After giving effect to the receipt of the net proceeds from our sale of 9,375,000 shares of common stock in this offering at the public offering price of $8.00 per share, after deducting underwriting discounts and commissions and estimated offering expenses payable by us, our as adjusted net tangible book value as of March 31, 2019, would have been $183.0 million, or $4.50 per share. This represents an immediate increase in as adjusted net tangible book value of $0.89 per share to our existing stockholders and immediate dilution of $3.50 per share to investors purchasing common stock in this offering.

The following table illustrates this dilution on a per share basis to new investors:

Public offering price per share			$	8.00
Net tangible book value per share as of March 31, 2019	$	3.61
Increase in net tangible book value per share attributable to new investors purchasing shares in this offering		0.89
As adjusted net tangible book value per share after this offering				4.50
Dilution in net tangible book value per share to new investors in this offering			$	3.50

If the underwriters exercise their option to purchase 1,406,250 additional shares of common stock in full at the public offering price of $8.00 per share, the as adjusted net tangible book value per share of our common stock immediately after this offering would be $4.60 per share, representing an increase to existing stockholders of $0.99 per share, and immediate dilution of $3.40 per share to new investors in this offering.

The number of shares of our common stock that will be outstanding after this offering is based on 31,298,599 shares of common stock outstanding as of March 31, 2019, and excludes:

• 3,223,178 shares of common stock issuable upon the exercise of outstanding stock options as of March 31, 2019, with a weighted-average exercise price of $6.72 per share;

• 2,861,930 shares of common stock reserved for future issuance under our 2018 Equity Incentive Plan as of March 31, 2019, as well as any automatic increases in the number of shares of common stock reserved for future issuance under this plan; and

• 716,136 shares of common stock reserved for issuance under our 2018 Employee Stock Purchase Plan as of March 31, 2019, as well as any automatic increases in the number of shares of common stock reserved for future issuance under this plan.

In addition, to the extent any outstanding options are exercised, new investors would experience further dilution.

S-22

MATERIAL UNITED STATES FEDERAL INCOME TAX CONSEQUENCES

TONON-U.S. HOLDERS OF OUR COMMON STOCK

The following is a summary of the material U.S. federal income tax consequences tonon-U.S. holders (as defined below) of the acquisition, ownership and disposition of our common stock issued pursuant to this offering. This discussion is not a complete analysis of all potential U.S. federal income tax consequences relating thereto, does not address the potential application of the Medicare contribution tax on net investment income, and does not address any estate or gift tax consequences or any tax consequences arising under any state, local or foreign tax laws, or any other U.S. federal tax laws. This discussion is based on the Internal Revenue Code of 1986, as amended, or the Code, and applicable Treasury Regulations promulgated thereunder, judicial decisions and published rulings and administrative pronouncements of the Internal Revenue Service, or IRS, all as in effect as of the date hereof. These authorities are subject to differing interpretations and may change, possibly retroactively, resulting in U.S. federal income tax consequences different from those discussed below. We have not requested a ruling from the IRS with respect to the statements made and the conclusions reached in the following summary, and there can be no assurance that the IRS or a court will agree with such statements and conclusions.

This discussion is limited tonon-U.S. holders who purchase our common stock pursuant to this offering and who hold our common stock as a “capital asset” within the meaning of Section 1221 of the Code (generally, property held for investment). This discussion does not address all of the U.S. federal income tax consequences that may be relevant to a particular holder in light of such holder’s particular circumstances. This discussion also does not consider any specific facts or circumstances that may be relevant to holders subject to special rules under the U.S. federal income tax laws, including:

• certain former citizens or long-term residents of the United States;

• partnerships or other pass-through entities (and investors therein);

• “controlled foreign corporations;”

• “passive foreign investment companies;”

• corporations that accumulate earnings to avoid U.S. federal income tax;

• banks, financial institutions, investment funds, insurance companies, brokers, dealers or traders in securities;

• tax-exempt organizations and governmental organizations;

• tax-qualified retirement plans;

• persons subject to the alternative minimum tax;

• persons that own, or have owned, actually or constructively, more than 5% of our common stock;

• accrual-method taxpayers subject to special tax accounting rules under Section 451(b) of the Code;

• persons who have elected to mark securities to market; and

• persons holding our common stock as part of a hedging or conversion transaction or straddle, or a constructive sale, or other risk reduction strategy or integrated investment.

If an entity or arrangement that is classified as a partnership for U.S. federal income tax purposes holds our common stock, the U.S. federal income tax treatment of a partner in the partnership will generally depend on the status of the partner and the activities of the partnership. Partnerships holding our common stock and the partners in such partnerships are urged to consult their tax advisors about the particular U.S. federal income tax consequences to them of holding and disposing of our common stock.

S-23

THIS DISCUSSION IS FOR INFORMATIONAL PURPOSES ONLY AND IS NOT TAX ADVICE. PROSPECTIVE INVESTORS SHOULD CONSULT THEIR TAX ADVISORS REGARDING THE PARTICULAR U.S. FEDERAL INCOME TAX CONSEQUENCES TO THEM OF ACQUIRING, OWNING AND DISPOSING OF OUR COMMON STOCK, AS WELL AS ANY TAX CONSEQUENCES ARISING UNDER ANY STATE, LOCAL OR FOREIGN TAX LAWS AND ANY OTHER U.S. FEDERAL TAX LAWS. IN ADDITION, SIGNIFICANT CHANGES IN U.S. FEDERAL INCOME TAX LAWS WERE RECENTLY ENACTED. YOU SHOULD ALSO CONSULT WITH YOUR TAX ADVISOR WITH RESPECT TO SUCH CHANGES IN U.S. TAX LAW AS WELL AS POTENTIAL CONFORMING CHANGES IN STATE TAX LAWS.

Definition ofNon-U.S. Holder

For purposes of this discussion, anon-U.S. holder is any beneficial owner of our common stock that is not a “U.S. person” or a partnership (including any entity or arrangement treated as a partnership) for U.S. federal income tax purposes. A U.S. person is any person that, for U.S. federal income tax purposes, is or is treated as any of the following:

• an individual who is a citizen or resident of the United States;

• a corporation (or entity treated as a corporation for U.S. federal income tax purposes) created or organized under the laws of the United States, any state thereof or the District of Columbia;

• an estate, the income of which is subject to U.S. federal income tax regardless of its source; or

• a trust (1) whose administration is subject to the primary supervision of a U.S. court and which has one or more U.S. persons who have the authority to control all substantial decisions of the trust or (2) that has a valid election in effect under applicable Treasury Regulations to be treated as a U.S. person.

Distributions on Our Common Stock

As described under the section titled “Dividend Policy,” we have not paid and do not anticipate paying dividends. However, if we make cash or other property distributions on our common stock, such distributions will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Amounts not treated as dividends for U.S. federal income tax purposes will constitute a return of capital and will first be applied against and reduce a holder’s tax basis in our common stock, but not below zero. Any excess will be treated as gain realized on the sale or other disposition of our common stock and will be treated as described under the section titled “—Gain on Disposition of Our Common Stock” below.

Subject to the discussions below regarding effectively connected income, backup withholding and Sections 1471 through 1474 of the Code (commonly referred to as FATCA), dividends paid to anon-U.S. holder of our common stock generally will be subject to U.S. federal withholding tax at a rate of 30% of the gross amount of the dividends or such lower rate specified by an applicable income tax treaty. To receive the benefit of a reduced treaty rate, anon-U.S. holder must furnish us or our paying agent with a valid IRS FormW-8BEN or IRS FormW-8BEN-E (or applicable successor form) and satisfy applicable certification and other requirements. This certification must be provided to us or our paying agent before the payment of dividends and must be updated periodically. If thenon-U.S. holder holds the stock through a financial institution or other agent acting on thenon-U.S. holder’s behalf, thenon-U.S. holder will be required to provide appropriate documentation to the agent, which then will be required to provide certification to us or our paying agent, either directly or through other intermediaries.

Non-U.S. holders that do not provide the required certification on a timely basis, but that qualify for a reduced treaty rate, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS.

S-24

If anon-U.S. holder holds our common stock in connection with the conduct of a trade or business in the United States, and dividends paid on our common stock are effectively connected with such holder’s U.S. trade or business (and are attributable to such holder’s permanent establishment in the United States if required by an applicable tax treaty), thenon-U.S. holder will be exempt from U.S. federal withholding tax. To claim the exemption, thenon-U.S. holder must generally furnish a valid IRS FormW-8ECI (or applicable successor form) to the applicable withholding agent.

However, any such effectively connected dividends paid on our common stock generally will be subject to U.S. federal income tax on a net income basis at the regular U.S. federal income tax rates in the same manner as if such holder were a resident of the United States. Anon-U.S. holder that is a foreign corporation also may be subject to an additional branch profits tax equal to 30% (or such lower rate specified by an applicable income tax treaty) of its effectively connected earnings and profits for the taxable year, as adjusted for certain items.Non-U.S. holders should consult their tax advisors regarding any applicable income tax treaties that may provide for different rules.

Gain on Disposition of Our Common Stock

Subject to the discussions below regarding backup withholding and FATCA, anon-U.S. holder generally will not be subject to U.S. federal income tax on any gain realized on the sale or other disposition of our common stock, unless:

• the gain is effectively connected with thenon-U.S. holder’s conduct of a trade or business in the United States, and if required by an applicable income tax treaty, is attributable to a permanent establishment maintained by thenon-U.S. holder in the United States;

• thenon-U.S. holder is a nonresident alien individual present in the United States for 183 days or more during the taxable year of the disposition, and certain other requirements are met; or

• our common stock constitutes a “United States real property interest” by reason of our status as a United States real property holding corporation, or USRPHC, for U.S. federal income tax purposes at any time within the shorter of the five-year period preceding the disposition or thenon-U.S. holder’s holding period for our common stock, and our common stock is not regularly traded on an established securities market during the calendar year in which the sale or other disposition occurs.

Determining whether we are a USRPHC depends on the fair market value of our U.S. real property interests relative to the fair market value of our other trade or business assets and our foreign real property interests. We believe that we are not currently and do not anticipate becoming a USRPHC for U.S. federal income tax purposes, although there can be no assurance we will not in the future become a USRPHC. If we are a USRPHC and either our common stock is not regularly traded on an established securities market or a non-U.S. holder holds, or is treated as holding, more than 5% of our outstanding common stock, directly or indirectly, during the applicable testing period, such non-U.S. holder will generally be taxed on any gain in the same manner as gain that is effectively connected with the conduct of a U.S. trade or business, except that the branch profits tax generally will not apply. If we are a USRPHC and our common stock is not regularly traded on an established securities market, a non-U.S. holder’s proceeds received on the disposition of shares will also generally be subject to withholding at a rate of 15%. Prospective investors are encouraged to consult their own tax advisors regarding the possible consequences to them if we are, or were to become, a USRPHC.

Gain described in the first bullet point above generally will be subject to U.S. federal income tax on a net income basis at the regular U.S. federal income tax rates in the same manner as if such holder were a resident of the United States. Anon-U.S. holder that is a foreign corporation also may be subject to an additional branch profits tax equal to 30% (or such lower rate specified by an applicable income tax treaty) of its effectively connected earnings and profits for the taxable year, as adjusted for certain items. Gain described in the second bullet point above will be subject to U.S. federal income tax at a flat 30% rate (or such lower rate specified by an applicable income tax treaty), but may be offset by certain U.S.-source capital losses (even though the individual

S-25

is not considered a resident of the United States), provided that thenon-U.S. holder has timely filed U.S. federal income tax returns with respect to such losses.Non-U.S. holders should consult their tax advisors regarding any applicable income tax treaties that may provide for different rules.

Information Reporting and Backup Withholding

Annual reports are required to be filed with the IRS and provided to eachnon-U.S. holder indicating the amount of dividends on our common stock paid to such holder and the amount of any tax withheld with respect to those dividends. These information reporting requirements apply even if no withholding was required because the dividends were effectively connected with the holder’s conduct of a U.S. trade or business, or withholding was reduced or eliminated by an applicable income tax treaty. This information also may be made available under a specific treaty or agreement with the tax authorities in the country in which thenon-U.S. holder resides or is established. Backup withholding, currently at a 24% rate, generally will not apply to payments to anon-U.S. holder of dividends on or the gross proceeds of a disposition of our common stock provided thenon-U.S. holder furnishes the required certification for itsnon-U.S. status, such as by providing a valid IRS FormW-8BEN, IRS FormW-8BEN-E or IRS FormW-8ECI, or certain other requirements are met. Backup withholding may apply if the payor has actual knowledge, or reason to know, that the holder is a U.S. person who is not an exempt recipient.

Backup withholding is not an additional tax. If any amount is withheld under the backup withholding rules, thenon-U.S. holder should consult with a U.S. tax advisor regarding the possibility of and procedure for obtaining a refund or a credit against thenon-U.S. holder’s U.S. federal income tax liability, if any.

Withholding on Foreign Entities

FATCA imposes a U.S. federal withholding tax of 30% on certain payments made to a “foreign financial institution” (as specially defined under these rules) unless such institution enters into an agreement with the U.S. government to withhold on certain payments and to collect and provide to the U.S. tax authorities substantial information regarding certain U.S. account holders of such institution (which includes certain equity and debt holders of such institution, as well as certain account holders that are foreign entities with U.S. owners) or an exemption applies. FATCA also generally will impose a U.S. federal withholding tax of 30% on certain payments made to anon-financial foreign entity unless such entity provides the withholding agent a certification identifying certain direct and indirect U.S. owners of the entity or an exemption applies. An intergovernmental agreement between the United States and an applicable foreign country may modify these requirements. Under certain circumstances, anon-U.S. holder might be eligible for refunds or credits of such taxes. FATCA currently applies to dividends paid on our common stock. Under applicable Treasury Regulations and administrative guidance, withholding under FATCA would have applied to payments of gross proceeds from the sale or other disposition of stock on or after January 1, 2019, but under proposed regulations (the preamble to which specifies that taxpayers are permitted to rely on such proposed regulations pending finalization), no withholding would apply with respect to payments of gross proceeds.

Prospective investors are encouraged to consult with their own tax advisors regarding the possible implications of this legislation on their investment in our common stock.

S-26

UNDERWRITERS

Under the terms and subject to the conditions in an underwriting agreement dated the date of this prospectus supplement, the underwriters named below, for whom Morgan Stanley & Co. LLC and Credit Suisse Securities (USA) LLC are acting as representatives, have severally agreed to purchase, and we have agreed to sell to them, severally, the number of shares of common stock indicated below:

Name	Number of Shares
Morgan Stanley & Co. LLC		4,031,250
Credit Suisse Securities (USA) LLC		3,281,250
Guggenheim Securities, LLC		937,500
Canaccord Genuity LLC		750,000
H.C. Wainwright & Co., LLC		187,500
Roth Capital Partners, LLC		187,500
Total		9,375,000

The underwriters and the representatives are collectively referred to as the “underwriters” and the “representatives,” respectively. The underwriters are offering the shares of common stock subject to their acceptance of the shares from us and subject to prior sale. The underwriting agreement provides that the obligations of the several underwriters to pay for and accept delivery of the shares of common stock offered by this prospectus supplement are subject to the approval of certain legal matters by their counsel and to certain other conditions. The underwriters are obligated to take and pay for all of the shares of common stock offered by this prospectus supplement if any such shares are taken. However, the underwriters are not required to take or pay for the shares covered by the underwriters’ option to purchase additional shares described below.

The underwriters initially propose to offer part of the shares of common stock directly to the public at the offering price listed on the cover page of this prospectus supplement and part to certain dealers at a price that represents a concession not in excess of $0.288 per share under the public offering price. After the initial offering of the shares of common stock, the offering price and other selling terms may from time to time be varied by the representatives.

We have granted to the underwriters an option, exercisable for 30 days from the date of this prospectus supplement, to purchase up to 1,406,250 additional shares of common stock at the public offering price listed on the cover page of this prospectus supplement, less underwriting discounts and commissions. To the extent the option is exercised, each underwriter will become obligated, subject to certain conditions, to purchase about the same percentage of the additional shares of common stock as the number listed next to the underwriter’s name in the preceding table bears to the total number of shares of common stock listed next to the names of all underwriters in the preceding table.

Jeffrey W. Bird, a member of our board of directors, and Bluebird Ventures, his affiliate, as well as entities affiliated with Lightspeed Venture Partners, an existing stockholder, and the Leukemia & Lymphoma Society, Inc., a strategic collaborator and existing stockholder that had submitted indications of interest, have agreed to purchase an aggregate of 1,750,000 shares of our common stock in this offering at the initial offering price.

S-27

The following table shows the per share and total public offering price, underwriting discounts and commissions, and proceeds before expenses to us. These amounts are shown assuming both no exercise and full exercise of the underwriters’ option to purchase up to an additional 1,406,250 shares of common stock.

Name	Per Share		Total
			No Exercise		Full Exercise
Public offering price	$	8.00	$	75,000,000	$	86,250,000
Underwriting discounts and commissions to be paid by us	$	0.48	$	4,500,000	$	5,175,000
Proceeds, before expenses, to us	$	7.52	$	70,500,000	$	81,075,000

The estimated offering expenses payable by us, exclusive of the underwriting discounts and commissions, are approximately $400,000. The underwriters have agreed to reimburse us for certain expenses incurred by us in connection with this offering. We have agreed to reimburse the underwriters for expenses of up to $25,000 relating to clearance of this offering with the Financial Industry Regulatory Authority, Inc. and compliance with state securities or “blue sky” laws.

Our common stock is listed on The Nasdaq Global Select Market under the trading symbol “FTSV.”

We and all of our directors and officers and certain holders of our common stock have agreed that, without the prior written consent of the representatives on behalf of the underwriters, we and they will not, during the period ending on and including the 90th day after the date of this prospectus supplement (the “restricted period”):

• offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend or otherwise transfer or dispose of, directly or indirectly, any shares of common stock or any securities convertible into or exercisable or exchangeable for shares of common stock;

• file any registration statement with the SEC relating to the offering of any shares of common stock or any securities convertible into or exercisable or exchangeable for common stock; or

• enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of the common stock;

whether any such transaction described above is to be settled by delivery of common stock or such other securities, in cash or otherwise. In addition, we and each such person agrees that, without the prior written consent of the representatives on behalf of the underwriters, we or such other person will not, during the restricted period, make any demand for, or exercise any right with respect to, the registration of any shares of common stock or any security convertible into or exercisable or exchangeable for common stock.

The restrictions described in the immediately preceding paragraph are subject to specified exceptions, including, without limitation:

• the sale of shares to the underwriters;

• the issuance by us of shares of common stock upon the exercise of an option or a warrant or the conversion of a security outstanding on the date of this prospectus supplement of which the underwriters have been advised in writing;

• transactions by any person other than us relating to shares of common stock or other securities acquired in this offering or in open market transactions after the closing of this offering;

• transfers of shares of common stock or any security convertible into common stock (a) as a bona fide gift or charitable contribution, (b) to an immediate family member or any trust for the direct or indirect benefit of the person subject to such restrictions or the immediate family of such person, (c) to a trustor or beneficiary of the trust or to the estate of a beneficiary of such trust, or (d) distributions of shares of common stock to limited partners, members, stockholders or holders of similar equity interests of the

S-28

party making such distribution or to direct or indirect subsidiaries of such party, provided that (i) each donee or other distributee shall sign and deliver alock-up letter substantially in the form attached as an exhibit to the underwriting agreement and (ii) no filing under Section 16(a) of the Securities Exchange Act of 1934, as amended, or Exchange Act, reporting a reduction in beneficial ownership of shares of common stock, and no other public announcement or filing, shall be required or shall be voluntarily made during the restricted period;

• in connection with the disposition or transfer of shares of common stock or any security convertible into common stock to us upon the “net” or “cashless” exercise of stock options or other equity awards outstanding as of the date of this prospectus supplement and granted pursuant to an employee benefit plan described in this prospectus supplement, provided that (i) such shares of common stock received upon exercise shall continue to be subject to the restrictions on transfer set forth in thelock-up agreement and (ii) no filing under Section 16(a) of the Exchange Act and no other public announcement or filing shall be required or voluntarily made during the restricted period;

• the exercise solely with cash of stock options outstanding as of the date of this prospectus supplement granted under an employee benefit plan or stock purchase plan described in this prospectus supplement, provided that (i) the shares received upon exercise shall continue to be subject to the restrictions on transfer set forth in thelock-up agreement, (ii) if required, any public report or filing under Section 16(a) of the Exchange Act shall clearly indicate in the footnotes thereto that the filing relates to the exercise of a stock option, that no shares were sold by the reporting person and that the shares received upon exercise are subject to alock-up agreement with the underwriters, and (iii) no other public announcement or filing shall be required or voluntarily made during the restricted period;

• transfers of shares of common stock or other securities to us in connection with a repurchase by us pursuant to a repurchase right arising upon the termination of the transferee’s employment with us pursuant to contractual agreements with us, provided that (i) any filing required by Section 16(a) of the Exchange Act shall clearly indicate in the footnotes thereto that such transfer is being made pursuant to such repurchase right under such agreement and (ii) no other public announcement or filing shall be required or voluntarily made during the restricted period;

• transfers by operation of law pursuant to a qualified domestic order or in connection with a divorce settlement, provided that (i) any filing required by Section 16(a) of the Exchange Act shall clearly indicate in the footnotes thereto that such transfer is being made pursuant to such court order and that such shares remain subject to alock-up agreement with the underwriters, and (ii) no other public announcement or filing shall be required or voluntarily made during the restricted period;

• transfers of shares of our common stock or other securities pursuant to a bona fide third-party tender offer, merger, consolidation or other similar transaction made to all holders of our common stock involving a change of control of our company that has been approved by our board of directors, provided that in the event that such tender offer, merger, consolidation or other such transaction is not completed, the securities shall continue to be subject to the restrictions on transfer set forth in thelock-up agreement;

• the establishment or amendment of a trading plan pursuant toRule 10b5-1 under the Exchange Act for the transfer of shares of common stock, provided that (i) such plan does not provide for the transfer of common stock during the restricted period and (ii) to the extent a public announcement or filing under the Exchange Act, if any, is required or voluntarily made regarding the establishment of such plan, such announcement or filing shall include a statement to the effect that no transfer of common stock may be made under such plan during the restricted period; and

• the sale of shares of our common stock pursuant to a10b5-1 trading plan established prior to the execution of the relevantlock-up agreement and not amended or modified during the restricted period, provided that any filing required by Section 16 of the Exchange Act made in connection with any such sale during the restricted period shall state that such sales have been executed under a10b5-1 trading plan.

S-29

The representatives, in their sole discretion, may release the common stock and other securities subject to thelock-up agreements described above in whole or in part at any time with or without notice.

In order to facilitate the offering of the common stock, the underwriters may engage in transactions that stabilize, maintain or otherwise affect the price of the common stock. Specifically, the underwriters may sell more shares than they are obligated to purchase under the underwriting agreement, creating a short position. A short sale is covered if the short position is no greater than the number of shares available for purchase by the underwriters under the option to purchase additional shares described above. The underwriters can close out a covered short sale by exercising such option or purchasing shares in the open market. In determining the source of shares to close out a covered short sale, the underwriters will consider, among other things, the open market price of shares compared to the price available under such option. The underwriters may also sell shares in excess of such option, creating a naked short position. The underwriters must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the common stock in the open market after pricing that could adversely affect investors who purchase in this offering. As an additional means of facilitating this offering, the underwriters may bid for, and purchase, shares of common stock in the open market to stabilize the price of the common stock. These activities may raise or maintain the market price of the common stock above independent market levels or prevent or retard a decline in the market price of the common stock. The underwriters are not required to engage in these activities and may end any of these activities at any time.

We and the underwriters have agreed to indemnify each other against certain liabilities, including liabilities under the Securities Act.

A prospectus in electronic format may be made available on websites maintained by one or more underwriters, or selling group members, if any, participating in this offering. The representatives may agree to allocate a number of shares of common stock to underwriters for sale to their online brokerage account holders. Internet distributions will be allocated by the representatives to underwriters that may make Internet distributions on the same basis as other allocations.

Other Relationships

The underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. Certain of the underwriters and their respective affiliates have, from time to time, performed, and may in the future perform, various financial advisory and investment banking services for us, for which they received or will receive customary fees and expenses.

In addition, in the ordinary course of their various business activities, the underwriters and their respective affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers and may at any time hold long and short positions in such securities and instruments. Such investment and securities activities may involve our securities and instruments. The underwriters and their respective affiliates may also make investment recommendations or publish or express independent research views in respect of such securities or instruments and may at any time hold, or recommend to clients that they acquire, long or short positions in such securities and instruments.

Selling Restrictions

Canada

The shares of our common stock may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument45-106 Prospectus Exemptions or

S-30

subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the shares of our common stock must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 (or, in the case of securities issued or guaranteed by the government of anon-Canadian jurisdiction, section 3A.4) of National Instrument33-105 Underwriting Conflicts(NI 33-105), the underwriters are not required to comply with the disclosure requirements ofNI 33-105 regarding underwriter conflicts of interest in connection with this offering.

European Economic Area

In relation to each Member State of the European Economic Area which has implemented the Prospectus Directive (each, a “Relevant Member State”) an offer to the public of any shares of our common stock may not be made in that Relevant Member State, except that an offer to the public in that Relevant Member State of any shares of our common stock may be made at any time under the following exemptions under the Prospectus Directive, if they have been implemented in that Relevant Member State:

(a)    to any legal entity which is a qualified investor as defined in the Prospectus Directive;

(b)    to fewer than 100 or, if the Relevant Member State has implemented the relevant provision of the 2010 PD Amending Directive, 150, natural or legal persons (other than qualified investors as defined in the Prospectus Directive), as permitted under the Prospectus Directive, subject to obtaining the prior consent of the representatives for any such offer; or

(c)    in any other circumstances falling within Article 3(2) of the Prospectus Directive, provided that no such offer of shares of our common stock shall result in a requirement for the publication by us or any underwriter of a prospectus pursuant to Article 3 of the Prospectus Directive.

For the purposes of this provision, the expression an “offer to the public” in relation to any shares of our common stock in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and any shares of our common stock to be offered so as to enable an investor to decide to purchase any shares of our common stock, as the same may be varied in that Member State by any measure implementing the Prospectus Directive in that Member State, the expression “Prospectus Directive” means Directive 2003/71/EC (and amendments thereto, including the 2010 PD Amending Directive, to the extent implemented in the Relevant Member State), and includes any relevant implementing measure in the Relevant Member State, and the expression “2010 PD Amending Directive” means Directive 2010/73/EU.

United Kingdom

Each underwriter has represented and agreed that:

(a)    it has only communicated or caused to be communicated and will only communicate or cause to be communicated an invitation or inducement to engage in investment activity (within the meaning of Section 21 of the Financial Services and Markets Act 2000, or FSMA) received by it in connection with the issue or sale of the shares of our common stock in circumstances in which Section 21(1) of the FSMA does not apply to us; and

S-31

(b)    it has complied and will comply with all applicable provisions of the FSMA with respect to anything done by it in relation to the shares of our common stock in, from or otherwise involving the United Kingdom.

Hong Kong

The shares of common stock have not been offered or sold and will not be offered or sold in Hong Kong, by means of any document, other than (a) to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) in other circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance. No advertisement, invitation or document relating to the shares of common stock has been or may be issued or has been or may be in the possession of any person for the purposes of issuance, whether in Hong Kong or elsewhere, which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) other than with respect to shares of common stock which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” as defined in the Securities and Futures Ordinance and any rules made under that Ordinance.

S-32

LEGAL MATTERS

Cooley LLP, Palo Alto, California, will pass upon the validity of the shares of common stock offered by this prospectus supplement and the accompanying prospectus. As of the date of this prospectus supplement, Cooley LLP beneficially owns 25,200 shares of our common stock. The underwriters are being represented in connection with this offering by Davis Polk & Wardwell LLP, Menlo Park, California.

EXPERTS

Ernst & Young LLP, independent registered public accounting firm, has audited our financial statements included in our Annual Report onForm 10-K for the year ended December 31, 2018, as set forth in their report, which is incorporated by reference in this prospectus supplement and elsewhere in the registration statement. Our financial statements are incorporated by reference in reliance on Ernst & Young LLP’s report, given on their authority as experts in accounting and auditing.

WHERE YOU CAN FIND ADDITIONAL INFORMATION

We have filed with the SEC a registration statement onForm S-3 under the Securities Act, with respect to the shares of common stock being offered by this prospectus supplement. This prospectus supplement and the accompanying prospectus, which constitutes part of the registration statement, do not contain all of the information in the registration statement and its exhibits. For further information with respect to our company and the common stock offered by this prospectus supplement and the accompanying prospectus, we refer you to the registration statement and its exhibits. Statements contained in this prospectus supplement and the accompanying prospectus as to the contents of any contract or any other document referred to are not necessarily complete, and in each instance, we refer you to the copy of the contract or other document filed as an exhibit to the registration statement. Each of these statements is qualified in all respects by this reference.

You can read our SEC filings, including the registration statement, over the internet at the SEC’s website at http://www.sec.gov. We are subject to the information reporting requirements of the Exchange Act, and file reports, proxy statements and other information with the SEC. These reports, proxy statements and other information are available for inspection on the website of the SEC referred to above. We also maintain a website at www.fortyseveninc.com, at which you may access these materials free of charge as soon as reasonably practicable after they are electronically filed with, or furnished to, the SEC. The information contained in, or that can be accessed through, our website is not part of, and is not incorporated into, this prospectus supplement.

S-33

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to “incorporate by reference” information from other documents that we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus supplement and the accompanying prospectus. Information in this prospectus supplement supersedes information incorporated by reference that we filed with the SEC prior to the date of this prospectus supplement, while information that we file later with the SEC will automatically update and supersede the information in this prospectus supplement. We incorporate by reference into this prospectus supplement and the accompanying prospectus the information or documents listed below that we have filed with the SEC (Commission FileNo. 001-38554):

• our Annual Report onForm10-K for the year ended December 31, 2018, filed with the SEC on March 28, 2019;

• the information specifically incorporated by reference in our Annual Report onForm10-K for the year ended December 31, 2018, from ourDefinitive Proxy Statement on Schedule 14A relating to our 2019 Annual Meeting of Stockholders, which was filed with the SEC on April 26, 2019, as supplemented byadditional materials filed with the SEC on May 20, 2019;

• our Current Reports on Form8-K filed with the SEC onJanuary 29, 2019,March 28, 2019 (Item 5.02),April 30, 2019,May  2, 2019,June 18, 2019 andJuly 11, 2019;

• our Quarterly Report onForm10-Q for the three months ended March 31, 2019, filed with the SEC on May 13, 2019; and

• the description of our common stock set forth in our registration statement onForm8-A, filed with the SEC on June 22, 2018, including any amendments thereto or reports filed for the purposes of updating this description.

Any documents we file with the SEC pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this prospectus supplement and prior to the termination of the offering of our common stock to which this prospectus supplement relates will automatically be deemed to be incorporated by reference into this prospectus supplement and to be part hereof from the date of filing those documents. We are not, however, incorporating by reference any documents or portions thereof that are not deemed “filed” with the SEC, including any information furnished pursuant to Item 2.02 or Item 7.01 of Form8-K or related exhibits furnished pursuant to Item 9.01 of Form8-K. Any statements in any such future filings will automatically be deemed to modify and supersede any information in any document we previously filed with the SEC that is incorporated or deemed to be incorporated herein by reference to the extent that statements in the later filed document modify or replace such earlier statements.

You can request a copy of these filings, at no cost, by writing or telephoning us at the following address or telephone number:

Forty Seven, Inc.

Attn: Investor Relations

1490 O’Brien Drive, Suite A

Menlo Park, California 94025

(650)352-4150

S-34

PROSPECTUS

$250,000,000

Common Stock

Preferred Stock

Debt Securities

Warrants

We may, from time to time, offer and sell up to $250,000,000 of any combination of the securities described in this prospectus, either individually or in combination, at prices and on terms described in one or more supplements to this prospectus. We may also offer common stock or preferred stock upon conversion of debt securities, or common stock upon conversion of preferred stock, or common stock, preferred stock or debt securities upon exercise of warrants.

This prospectus describes some of the general terms that may apply to an offering of our securities. We will provide the specific terms of these offerings and securities in one or more supplements to this prospectus. We may also authorize one or more free writing prospectuses to be provided to you in connection with these offerings. The prospectus supplement and any related free writing prospectus may also add, update or change information contained in this prospectus. You should carefully read this prospectus, the applicable prospectus supplement and any related free writing prospectus, as well as the documents incorporated by reference, before buying any of the securities being offered.

Securities may be sold by us to or through underwriters or dealers, directly to purchasers or through agents designated from time to time. For additional information on the methods of sale, you should refer to the section titled “Plan of Distribution” in this prospectus and in the applicable prospectus supplement. If any underwriters are involved in the sale of any securities with respect to which this prospectus is being delivered, the names of such underwriters and any applicable discounts or commissions and options to purchase additional securities will be set forth in a prospectus supplement. The price to the public of such securities and the net proceeds we expect to receive from such sale will also be set forth in a prospectus supplement.

Our common stock is listed on The Nasdaq Global Select Market under the trading symbol “FTSV.” On July 11, 2019, the closing price of our common stock was $10.30 per share. The applicable prospectus supplement will contain information, where applicable, as to other listings, if any, on The Nasdaq Global Select Market or other securities exchange of the securities covered by the prospectus supplement.

Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties described under the heading “Risk Factors” on page 6 of this prospectus and in our most recent Annual Report on Form10-K and Quarterly Report on Form10-Q incorporated by reference into this prospectus, the applicable prospectus supplement and in any free writing prospectuses we have authorized for use in connection with a specific offering, and under similar headings in the other documents that are incorporated by reference into this prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

We are an “emerging growth company” under the federal securities laws and are subject to reduced public company reporting requirements.

The date of this prospectus is July 12, 2019.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS	i
PROSPECTUS SUMMARY	1
RISK FACTORS	6
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS	7
USE OF PROCEEDS	9
DESCRIPTION OF CAPITAL STOCK	10
DESCRIPTION OF DEBT SECURITIES	17
DESCRIPTION OF WARRANTS	24
LEGAL OWNERSHIP OF SECURITIES	26
PLAN OF DISTRIBUTION	30
LEGAL MATTERS	32
EXPERTS	32
WHERE YOU CAN FIND MORE INFORMATION	32
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE	33

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement on FormS-3 that we filed with the Securities and Exchange Commission, or SEC, using a “shelf” registration process under the Securities Act of 1933, as amended, or the Securities Act. Under this shelf registration statement, we may sell from time to time in one or more offerings of common stock and preferred stock, various series of debt securities and/or warrants to purchase any of such securities, either individually or in combination with other securities, in one or more offerings described in this prospectus. This prospectus provides you with a general description of the securities we may offer.

Each time we sell any type or series of securities under this prospectus, we will provide a prospectus supplement that will contain more specific information about the terms of that offering. We may also authorize one or more free writing prospectuses to be provided to you that may contain material information relating to these offerings. We may also add, update or change in a prospectus supplement or free writing prospectus any of the information contained in this prospectus or in the documents we have incorporated by reference into this prospectus. This prospectus, together with the applicable prospectus supplement, any related free writing prospectus and the documents incorporated by reference into this prospectus and the applicable prospectus supplement, will include all material information relating to the applicable offering. You should carefully read both this prospectus and the applicable prospectus supplement and any related free writing prospectus, together with the additional information described under “Where You Can Find More Information,” before buying any of the securities being offered.

We have not authorized anyone to provide you with any information other than contained in, or incorporated by reference into, this prospectus and the applicable prospectus supplement, along with the information contained in any free writing prospectuses we have authorized for use in connection with a specific offering. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. This prospectus is an offer to sell only the securities offered hereby, but only under circumstances and in jurisdictions where it is lawful to do so. You should not assume that the information contained in or incorporated by reference in this prospectus or any prospectus supplement or in any such free writing prospectus

i

is accurate as of any date other than their respective dates. Our business, financial condition, results of operations and prospects may have changed since those dates.

This prospectus contains summaries of certain provisions contained in some of the documents described herein, but reference is made to the actual documents for complete information. All of the summaries are qualified in their entirety by the actual documents. Copies of some of the documents referred to herein have been filed, will be filed or will be incorporated by reference as exhibits to the registration statement of which this prospectus is a part, and you may obtain copies of those documents as described below under the section titled “Where You Can Find More Information.”

Unless the context indicates otherwise, as used in this prospectus, the terms “Forty Seven,” “Forty Seven, Inc.” “Company,” “we,” “us” and “our” refer to Forty Seven, Inc. and, where appropriate, its subsidiaries on a consolidated basis.

This prospectus and the information incorporated herein by reference include trademarks, service marks and trade names owned by us or other companies. All trademarks, service marks and trade names included or incorporated by reference into this prospectus, any applicable prospectus supplement or any related free writing prospectus are the property of their respective owners.

ii

PROSPECTUS SUMMARY

This summary highlights selected information contained elsewhere in this prospectus or incorporated by reference in this prospectus, and does not contain all of the information that you need to consider in making your investment decision. You should carefully read the entire prospectus, the applicable prospectus supplement and any related free writing prospectus, including the risks of investing in our securities discussed under the heading “Risk Factors” contained in this prospectus, the applicable prospectus supplement and any related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus. You should also carefully read the information incorporated by reference into this prospectus, including our financial statements, and the exhibits to the registration statement of which this prospectus is a part.

Forty Seven, Inc.

Overview

We are a clinical-stage immuno-oncology company focused on developing novel therapies to activate macrophages in the fight against cancer. We founded Forty Seven based on the insight that blocking CD47, a key signaling molecule that is overexpressed on cancer cells, renders tumors susceptible to macrophages. By harnessing macrophages, we believe that our lead product candidate, 5F9, can transform the treatment of cancer. 5F9 has demonstrated promising activity in multiple Phase 1b/2 clinical trials in which we have treated over 290 cancer patients with solid or hematologic tumors. In addition, we have two additional product candidates in preclinical development;FSI-189 an anti-SIRPα antibody andFSI-174 an anti-CKIT antibody.

We focus our efforts on targeting the CD47 pathway as a way to engage macrophages in fighting tumors. Macrophages function as first responders, swallowing foreign and abnormal cells, including cancer cells, and mobilizing other components of the immune system including T cells and antibodies. Cancer cells use CD47, a “don’t eat me” signal, in order to evade detection by the immune system and subsequent destruction by macrophages. Overexpression of CD47 is common to nearly all types of tumors and is also correlated with poor prognosis in multiple cancers including acute myelogenous leukemia, or AML, colorectal cancer, or CRC, gastric cancer, lung cancer,Non-Hodgkin’s lymphoma, or NHL, and ovarian cancer. Despite the central role of macrophages as cell-eating scavengers and first responders, the pharmaceutical industry is only beginning to bring this key group of cells into the fight against cancer.

Our company was founded by leading scientists at Stanford University who uncovered the fundamental role of CD47 in cancer evasion. Preclinical work performed in the laboratory of ourco-founder, Irving L. Weissman, at Stanford University and at Forty Seven demonstrated that:

• Blocking the CD47 “don’t eat me” signaling pathway leads to elimination of many types of tumors and increased survival;

• Boosting an “eat me” signal found on cancer cells using therapeutic antibodies results in a synergistic effect with blocking CD47;

• Macrophages digest cancer cells in a process called phagocytosis and present tumor-specific antigens that can activate T cells against the cancer, thus creating the potential for synergy with T cell checkpoint inhibitors; and

• Inducing an “eat me” signal on cancer cells using certain chemotherapies results in improved targeting of tumor cells which is synergistic with blocking CD47.

Our lead product candidate, 5F9, is a humanized IgG4 subclass monoclonal antibody against CD47 that is designed to interfere with recognition of CD47 by the SIRPα receptor on macrophages, thus blocking the “don’t

1

eat me” signal. The design of 5F9, combined with our proprietary dosing regimen, overcomes the toxicity limitations of previously tested anti-CD47 therapies developed by others. Across all study populations, 5F9 has been well tolerated with no maximum tolerated dose, or MTD, observed in any study despite dosing up to 45 mg/ kg. The most common treatment-associated effects observed to date were the expected CD47-mechanism-based effects on red blood cells, which led to a temporary and reversible anemia. Other reported treatment-related adverse events include infusion reactions, headache, fatigue, chills, fever and nausea. The majority of these adverse events were mild to moderate in severity and were generally easily managed. To date, there are no approved therapies that target the CD47 checkpoint of the innate immune system.

The targeting of CD47 to make cancer cells susceptible to macrophages, a component of the innate immune system, is analogous to the approach that has been applied with checkpoint inhibitors and T cells, a component of the adaptive immune system. Since their introduction in 2011, T cell checkpoint inhibitors have become frontline therapies for certain cancers and we estimate that they generated over $17 billion in sales in 2018. Despite the success of T cell checkpoint inhibitors, these therapies have been shown to be effective only in a subset of tumors, highlighting the need for additional therapies. Similar to the way cancer cells overexpress programmed death-ligand 1, orPD-L1, to avoid attack by T cells, cancer cells overexpress CD47 as a way to avoid destruction by macrophages. We believe targeting CD47 represents a compelling and analogous approach.

Corporate Information

We were incorporated in Delaware in 2014 as CD47 Sciences, Inc. Our principal executive offices are located at 1490 O’Brien Drive, Suite A, Menlo Park, California 94025, and our telephone number is(650) 352-4150.

Our website address is www.fortyseveninc.com. Information contained on, or that can be accessed through, our website is not incorporated by reference into this prospectus, and you should not consider information on our website to be part of this prospectus.

“Forty Seven,” the Forty Seven logo and other trademarks or service marks of Forty Seven appearing in this prospectus are our property. This prospectus contains additional trade names, trademarks, and service marks of other companies, which are the property of their respective owners. We do not intend our use or display of other companies’ trade names, trademarks or service marks to imply a relationship with, or endorsement or sponsorship of us by, these other companies.

Description of Securities

We may offer shares of our common stock and preferred stock, various series of debt securities and/or warrants to purchase any of such securities, either individually or in combination with other securities, from time to time under this prospectus, together with the applicable prospectus supplement and any related free writing prospectus, at prices and on terms to be determined at the time of any offering. We may also offer common stock, preferred stock and/or debt securities upon the exercise of warrants. This prospectus provides you with a general description of the securities we may offer. Each time we offer a type or series of securities under this prospectus, we will provide a prospectus supplement that will describe the specific amounts, prices and other important terms of the securities, including, to the extent applicable:

• designation or classification;

• aggregate principal amount or aggregate offering price;

• maturity date, if applicable;

• original issue discount, if any;

2

• rates and times of payment of interest or dividends, if any;

• redemption, conversion, exercise, exchange or sinking fund terms, if any;

• ranking;

• restrictive covenants, if any;

• voting or other rights, if any;

• conversion or exchange prices or rates, if any, and, if applicable, any provisions for changes to or adjustments in the conversion or exchange prices or rates and in the securities or other property receivable upon conversion or exchange; and

• material or special U.S. federal income tax considerations, if any.

The applicable prospectus supplement and any related free writing prospectus that we may authorize to be provided to you may also add, update or change any of the information contained in this prospectus or in the documents we have incorporated by reference.

We may sell the securities directly to investors or to or through agents, underwriters or dealers. We, and our agents or underwriters, reserve the right to accept or reject all or part of any proposed purchase of securities. If we do offer securities to or through agents or underwriters, we will include in the applicable prospectus supplement:

• the names of those agents or underwriters;

• applicable fees, discounts and commissions to be paid to them;

• details regarding over-allotment or other options, if any; and

• the net proceeds to us, if any.

Common Stock

We may issue shares of our common stock from time to time. The holders of common stock are entitled to one vote per share on all matters to be voted on by the stockholders. Subject to the preferences that may be applicable to any outstanding shares of preferred stock, the holders of common stock are entitled to receive ratably any dividends our board of directors declares out of funds legally available for the payment of dividends. If we are liquidated, dissolved or wound up, the holders of common stock are entitled to share pro rata all assets remaining after payment of liabilities and liquidation preferences of any outstanding shares of preferred stock. Holders of common stock have no preemptive rights or rights to convert their common stock into any other securities. There are no redemption or sinking fund provisions applicable to the common stock. In this prospectus, we have summarized certain general features of the common stock under the heading “Description of Capital Stock — Common Stock.” We urge you, however, to read the applicable prospectus supplement (and any related free writing prospectus that we may authorize to be provided to you) related to any common stock being offered.

Preferred Stock

We may issue shares of our preferred stock from time to time, in one or more series. Our board of directors will determine the designations, voting powers, preferences and rights of the preferred stock, as well as the qualifications, limitations or restrictions thereof, including dividend rights, conversion rights, preemptive rights, terms of redemption or repurchase, liquidation preferences, sinking fund terms and the number of shares constituting any series or the designation of any series, or the designation of such series, any or all of which may

3

be greater than the rights of our common stock. Convertible preferred stock will be convertible into our common stock or exchangeable for other securities. Conversion may be mandatory or at your option and would be at prescribed conversion rates.

If we sell any series of preferred stock under this prospectus, we will fix the designations, voting powers, preferences and rights of the preferred stock of each series we issue under this prospectus, as well as the qualifications, limitations or restrictions thereof, in the certificate of designation relating to that series. We will file as an exhibit to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of any certificate of designation that contains the terms of the series of preferred stock we are offering. In this prospectus, we have summarized certain general features of the preferred stock under “Description of Capital Stock — Preferred Stock.” We urge you, however, to read the applicable prospectus supplement (and any related free writing prospectus that we may authorize to be provided to you) related to the series of preferred stock being offered, as well as the complete certificate of designation that contains the terms of the applicable series of preferred stock.

Debt Securities

We may issue debt securities from time to time, in one or more series, as either senior or subordinated debt or as senior or subordinated convertible debt. The senior debt securities will rank equally with any other unsecured and unsubordinated debt. The subordinated debt securities will be subordinate and junior in right of payment, to the extent and in the manner described in the instrument governing the debt, to all of our senior indebtedness. Convertible or exchangeable debt securities will be convertible into or exchangeable for our common stock or our other securities. Conversion or exchange may be mandatory or optional (at our option or the holders’ option) and would be at prescribed conversion or exchange prices.

The debt securities will be issued under an indenture that we will enter into with a national banking association or other eligible party, as trustee. In this prospectus, we have summarized certain general features of the debt securities under the heading “Description of Debt Securities.” We urge you, however, to read the applicable prospectus supplement (and any related free writing prospectus that we may authorize to be provided to you) related to the series of debt securities being offered, as well as the complete indenture and any supplemental indentures that contain the terms of the debt securities. We have filed the form of indenture as an exhibit to the registration statement of which this prospectus is a part, and supplemental indentures and forms of debt securities containing the terms of the debt securities being offered will be filed as exhibits to the registration statement of which this prospectus is a part or will be incorporated by reference from reports that we file with the SEC.

Warrants

We may issue warrants for the purchase of common stock, preferred stock and/or debt securities in one or more series. We may issue warrants independently or in combination with common stock, preferred stock and/or debt securities offered by any prospectus supplement. In this prospectus, we have summarized certain general features of the warrants under the heading “Description of Warrants.” We urge you, however, to read the applicable prospectus supplement (and any related free writing prospectus that we may authorize to be provided to you) related to the particular series of warrants being offered, as well as the form of warrant and/or the warrant agreement and warrant certificate, as applicable, that contain the terms of the warrants. We have filed the forms of the warrant agreements and forms of warrant certificates containing the terms of the warrants that we may offer as exhibits to the registration statement of which this prospectus is a part. We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of warrant and/or the warrant agreement and warrant certificate, as applicable, that contain the terms of the particular series of warrants we are offering, and any supplemental agreements, before the issuance of such warrants.

4

Warrants may be issued under a warrant agreement that we enter into with a warrant agent. We will indicate the name and address of the warrant agent, if any, in the applicable prospectus supplement relating to a particular series of warrants.

Use of Proceeds

Except as described in any applicable prospectus supplement or in any free writing prospectuses we have authorized for use in connection with a specific offering, we currently intend to use the net proceeds from the sale of the securities offered by us hereunder, if any, to conduct our clinical trials, to fund continued research and development of 5F9 in several applications, to fund other research and development activities and for working capital and other general corporate purposes. See “Use of Proceeds” in this prospectus.

The Nasdaq Global Select Market Listing

Our common stock is listed on The Nasdaq Global Select Market under the symbol “FTSV.” The applicable prospectus supplement will contain information, where applicable, as to other listings, if any, on The Nasdaq Global Select Market or any other securities market or other exchange of the securities covered by the applicable prospectus supplement.

5

RISK FACTORS

Investing in our securities involves a high degree of risk. Before deciding whether to invest in our securities, you should consider carefully the risks and uncertainties described under the heading “Risk Factors” contained in the applicable prospectus supplement and any related free writing prospectus, and discussed under the section titled “Risk Factors” contained in our most recent Annual Report onForm 10-K and Quarterly Report onForm 10-Q, as well as any amendments thereto reflected in subsequent filings with the SEC, which are incorporated by reference into this prospectus in their entirety, together with other information in this prospectus, the documents incorporated by reference and any free writing prospectus that we may authorize for use in connection with a specific offering. The risks described in these documents are not the only ones we face, but those that we consider to be material. There may be other unknown or unpredictable economic, business, competitive, regulatory or other factors that could have material adverse effects on our future results. Past financial performance may not be a reliable indicator of future performance, and historical trends should not be used to anticipate results or trends in future periods. If any of these risks actually occurs, our business, financial condition, results of operations or cash flow could be seriously harmed. This could cause the trading price of our securities to decline, resulting in a loss of all or part of your investment. Please also read carefully the section below titled “Special Note Regarding Forward-Looking Statements.”

6

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the documents we have filed with the SEC that are incorporated by reference contain “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. These statements relate to future events or to our future operating or financial performance and involve known and unknown risks, uncertainties and other factors that may cause our actual results, performance or achievements to be materially different from any future results, performances or achievements expressed or implied by the forward-looking statements. All statements other than statements of historical facts contained in this prospectus, including statements regarding our future results of operations and financial condition, business strategy and plans and objectives of management for future operations, are forward-looking statements. In some cases, forward-looking statements may be identified by words such as “anticipate,” “believe,” “continue,” “could,” “design,” “estimate,” “expect,” “intend,” “may,” “plan,” “potentially,” “predict,” “project,” “should,” “will” or the negative of these terms or other similar expressions.

We have based these forward-looking statements largely on our current expectations and projections about future events and financial trends that we believe may affect our financial condition, results of operations, business strategy and financial needs. These forward-looking statements are subject to a number of known and unknown risks, uncertainties and assumptions, including risks described in the section titled “Risk Factors” and elsewhere in this prospectus and in our most recent Annual Report on Form10-K and Quarterly Report on Form10-Q, as well as any amendments thereto reflected in subsequent filings with the SEC, which are incorporated by reference into this prospectus in their entirety, together with other information in this prospectus, the documents incorporated by reference and any free writing prospectus that we may authorize for use in connection with a specific offering. These factors include, among other things:

• the success, cost and timing of our product development activities and clinical trials;

• our expectations about the timing of achieving regulatory approval and the cost of our development programs;

• our ability to obtain funding for our operations, including funding necessary to complete further development and commercialization of our product candidates;

• the commercialization of our product candidates, if approved;

• our plans to research, develop and commercialize our product candidates;

• our ability to maintain, expand, protect and enforce our intellectual property portfolio;

• our ability to operate our business without infringing, misappropriating or otherwise violating the intellectual property rights of third parties;

• our ability to attract collaborators with development, regulatory and commercialization expertise;

• our expectations regarding our ability to obtain and maintain intellectual property protection for our product candidate;

• our ability to reach agreements and work with third parties in connection with the development and commercialization of our product candidates;

• the size and growth potential of the markets for our product candidates, and our ability to serve those markets;

• the rate and degree of market acceptance of our product candidates;

• regulatory developments in the United States and foreign countries;

• our ability to contract with third-party suppliers and manufacturers and their ability to perform adequately;

7

• the success of competing therapies that are or may become available;

• our ability to attract and retain key scientific or management personnel;

• the accuracy of our estimates regarding expenses, future revenue, capital requirements and needs for additional financing;

• our expectations regarding the period during which we qualify as an emerging growth company under the JOBS Act; and

• our ability to maintain proper and effective internal controls.

These risks are not exhaustive. Other sections of this prospectus may include additional factors that could harm our business and financial performance. Moreover, we operate in a very competitive and rapidly changing environment. New risk factors emerge from time to time, and it is not possible for our management to predict all risk factors nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ from those contained in, or implied by, any forward-looking statements.

You should not rely upon forward-looking statements as predictions of future events. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements. Except as required by law, we undertake no obligation to update publicly any forward-looking statements for any reason after the date of this prospectus or to conform these statements to actual results or to changes in our expectations.

In addition, statements that “we believe” and similar statements reflect our beliefs and opinions on the relevant subject. These statements are based upon information available to us as of the date of this prospectus, and while we believe such information forms a reasonable basis for such statements, such information may be limited or incomplete, and our statements should not be read to indicate that we have conducted an exhaustive inquiry into, or review of, all relevant information. These statements are inherently uncertain and investors are cautioned not to unduly rely upon these statements.

You should read this prospectus and the documents that we reference in this prospectus and have filed as exhibits to the registration statement of which this prospectus is a part with the understanding that our actual future results, levels of activity, performance and achievements may be different from what we expect. We qualify all of our forward-looking statements by these cautionary statements.

8

USE OF PROCEEDS

Except as described in any applicable prospectus supplement or in any free writing prospectuses we have authorized for use in connection with a specific offering, we currently intend to use the net proceeds from the sale of the securities offered by us hereunder, if any, to conduct our clinical trials, to fund continued research and development of 5F9 in several applications, to fund other research and development activities and for working capital and other general corporate purposes.

9

DESCRIPTION OF CAPITAL STOCK

The description below of our capital stock and provisions of our amended and restated certificate of incorporation and amended and restated bylaws are summaries and are qualified by reference to the amended and restated certificate of incorporation and the amended and restated bylaws which are filed as exhibits to the registration statement of which this prospectus is part, and by the applicable provisions of Delaware law.

General

Our amended and restated certificate of incorporation authorizes us to issue up to 200,000,000 shares of common stock, $0.0001 par value per share, and 10,000,000 shares of preferred stock, $0.0001 par value per share.

As of March 31, 2019, there were 31,298,599 shares of common stock issued and outstanding, held by 87 stockholders of record, and no shares of preferred stock issued and outstanding.

Common Stock

Voting Rights

Each holder of common stock is entitled to one vote for each share on all matters submitted to a vote of the stockholders, including the election of directors. Under our amended and restated certificate of incorporation and amended and restated bylaws, our stockholders do not have cumulative voting rights. Because of this, the holders of a majority of the shares of common stock entitled to vote in any election of directors can elect all of the directors standing for election.

Dividend Rights

Subject to preferences that may apply to any then outstanding preferred stock, the holders of common stock are entitled to receive ratably those dividends, if any, as may be declared from time to time by the board of directors out of legally available funds. We do not anticipate paying any cash dividends in the foreseeable future.

Liquidation Rights

In the event of our liquidation, dissolution or winding up, holders of common stock will be entitled to share ratably in the net assets legally available for distribution to stockholders after the payment of all of our debts and other liabilities and the satisfaction of any liquidation preference granted to the holders of any then-outstanding shares of preferred stock.

Preemptive or Similar Rights

Holders of common stock have no preemptive, conversion or subscription rights and there are no redemption or sinking fund provisions applicable to the common stock. The rights, preferences and privileges of the holders of common stock are subject to, and may be adversely affected by, the rights of the holders of shares of any series of preferred stock that we may designate in the future.

Preferred Stock

Our board of directors is authorized, subject to limitations prescribed by Delaware law, to issue preferred stock in one or more series, to establish from time to time the number of shares to be included in each series and to fix the designation, powers, preferences and rights of the shares of each series and any of its qualifications, limitations or restrictions. Our board of directors can also increase or decrease the number of shares of any series,

10

but not below the number of shares of that series then outstanding, without any further vote or action by our stockholders. Our board of directors may authorize the issuance of preferred stock with voting or conversion rights that could adversely affect the voting power or other rights of the holders of the common stock common stock. The issuance of preferred stock, while providing flexibility in connection with financings, possible acquisitions and other corporate purposes, could, among other things, have the effect of delaying, deferring, discouraging or preventing a change in control of our company, may adversely affect the market price of our common stock and the voting and other rights of the holders of common stock, and may reduce the likelihood that common stock stockholders will receive dividend payments and payments upon liquidation.

We will fix the designations, voting powers, preferences and rights of the preferred stock of each series we issue under this prospectus, as well as the qualifications, limitations or restrictions thereof, in the certificate of designation relating to that series. We will file as an exhibit to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of any certificate of designation that contains the terms of the series of preferred stock we are offering. We will describe in the applicable prospectus supplement the terms of the series of preferred stock being offered, including, to the extent applicable:

• the title and stated value;

• the number of shares we are offering;

• the liquidation preference per share;

• the purchase price;

• the dividend rate, period and payment date and method of calculation for dividends;

• whether dividends will be cumulative ornon-cumulative and, if cumulative, the date from which dividends will accumulate;

• the procedures for any auction and remarketing, if applicable;

• the provisions for a sinking fund, if applicable;

• the provisions for redemption or repurchase, if applicable, and any restrictions on our ability to exercise those redemption and repurchase rights;

• any listing of the preferred stock on any securities exchange or market;

• whether the preferred stock will be convertible into our common stock, and, if applicable, the conversion price, or how it will be calculated, and the conversion period;

• whether the preferred stock will be exchangeable into debt securities, and, if applicable, the exchange price, or how it will be calculated, and the exchange period;

• voting rights of the preferred stock;

• preemptive rights, if any;

• restrictions on transfer, sale or other assignment;

• whether interests in the preferred stock will be represented by depositary shares;

• a discussion of any material or special U.S. federal income tax considerations applicable to the preferred stock;

• the relative ranking and preferences of the preferred stock as to dividend rights and rights if we liquidate, dissolve or wind up our affairs;

• any limitations on the issuance of any class or series of preferred stock ranking senior to or on a parity with the series of preferred stock as to dividend rights and rights if we liquidate, dissolve or wind up our affairs; and

• any other specific terms, preferences, rights or limitations of, or restrictions on, the preferred stock.

11

The transfer agent for each series of preferred stock will be described in the applicable prospectus supplement.

Registration Rights

We entered into an Investor Rights Agreement, or IRA, in connection with the issuance of our Series B preferred stock in October 2017, which provides registration rights to certain holders of shares of common stock. These shares are referred to as registrable securities. The following summary discusses certain material provisions of the IRA.

The registration of shares of common stock pursuant to the exercise of registration rights described below would enable the holders to trade these shares without restriction under the Securities Act of 1933, as amended, or the Securities Act, when the applicable registration statement is declared effective. We will pay the registration expenses (other than underwriting discounts, selling commissions and stock transfer taxes) of the shares registered pursuant to the demand, piggyback and FormS-3 registrations described below.

Generally, in an underwritten offering, if we determine in good faith in consultation with the underwriters, we have the right, subject to specified conditions, to limit the number of shares the holders may include. The demand, piggyback and FormS-3 registration rights described below will terminate in July 2021.

Demand Registration Rights

The holders of the registrable securities are entitled to certain demand registration rights. Upon the written request of the holders of more than 50% of our registrable securities then outstanding that we file a registration statement under the Securities Act covering at least 50% of the registrable securities then outstanding, or a lesser percent if the anticipated aggregate offering price, net of underwriting discounts and commissions, would exceed $7,500,000, we are obligated to register the sale of all registrable securities that the holders may request in writing to be registered. We are required to effect no more than two registration statements that are declared or ordered effective. We may postpone the filing of a registration statement for up to 120 days once in a12-month period if in the good faith judgment of our board of directors such registration would be seriously detrimental to us.

Piggyback Registration Rights

In connection with the filing of the registration statement of which this prospectus forms a part, the holders of registrable securities were entitled to, and the necessary percentage of holders waived, their rights to include their shares of registrable securities in the registration statement of which this prospectus forms a part. If we propose to register for offer and sale any of our securities under the Securities Act in another offering, either for our own account or for the account of other security holders, the holders of these shares will be entitled to certain “piggyback” registration rights allowing them to include their shares in such registration, subject to certain marketing and other limitations. As a result, whenever we propose to file a registration statement under the Securities Act, including a registration statement on FormS-3 as discussed below, other than with respect to a registration relating to any of our equity incentive plans, stock purchase or similar plan, a transaction under Rule 145 of the Securities Act or a registration related to stock issued upon conversion of debt securities, the holders of these shares are entitled to notice of the registration and have the right, subject to limitations that we, in consultation with the underwriters, may impose on the number of shares included in the registration, to include their shares in the registration.

FormS-3 Registration Rights

The holders of the registrable securities can request that we register all or a portion of their shares on FormS-3 if we are eligible to file a registration statement on FormS-3 and the aggregate price to the public of the

12

shares offered is in excess of $1.0 million (net of underwriting discounts and commissions, if any). We are required to effect no more than two FormS-3 registration statements that are declared or ordered effective in any12-month period. We may postpone the filing of a registration statement for up to 120 days not more than twice in a12-month period if in the good faith judgment of our board of directors such registration would be seriously detrimental to us.

Anti-Takeover Provisions

Section 203 of the Delaware General Corporation Law

We are subject to Section 203 of the Delaware General Corporation Law, which prohibits a Delaware corporation from engaging in any business combination with any interested stockholder for a period of three years after the date that such stockholder became an interested stockholder, with the following exceptions:

• before such date, the board of directors of the corporation approved either the business combination or the transaction that resulted in the stockholder becoming an interested stockholder;

• upon closing of the transaction that resulted in the stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of the corporation outstanding at the time the transaction began, excluding for purposes of determining the voting stock outstanding (but not the outstanding voting stock owned by the interested stockholder) those shares owned (i) by persons who are directors and also officers and (ii) employee stock plans in which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer; or

• on or after such date, the business combination is approved by the board of directors and authorized at an annual or special meeting of the stockholders, and not by written consent, by the affirmative vote of at least 66 2/3% of the outstanding voting stock that is not owned by the interested stockholder.

• In general, Section 203 defines a “business combination” to include the following:

• any merger or consolidation involving the corporation and the interested stockholder;

• any sale, transfer, pledge or other disposition of 10% or more of the assets of the corporation involving the interested stockholder;

• subject to certain exceptions, any transaction that results in the issuance or transfer by the corporation of any stock of the corporation to the interested stockholder;

• any transaction involving the corporation that has the effect of increasing the proportionate share of the stock or any class or series of the corporation beneficially owned by the interested stockholder; or

• the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits by or through the corporation.

In general, Section 203 defines an “interested stockholder” as an entity or person who, together with the person’s affiliates and associates, beneficially owns, or within three years prior to the time of determination of interested stockholder status did own, 15% or more of the outstanding voting stock of the corporation.

A Delaware corporation may “opt out” of these provisions with an express provision in its original certificate of incorporation or an express provision in its amended and restated certificate of incorporation or amended and restated bylaws resulting from a stockholders’ amendment approved by at least a majority of the outstanding voting shares. We have not opted out of these provisions. As a result, mergers or other takeover or change in control attempts of us may be discouraged or prevented.

13

Certificate of Incorporation and Bylaws

Among other things, our amended and restated certificate of incorporation and amended and restated bylaws:

• permit our board of directors to issue up to 10,000,000 shares of preferred stock, with any rights, preferences and privileges as they may designate, including the right to approve an acquisition or other change of control;

• provide that the authorized number of directors may be changed only by resolution of our board of directors;

• provide that our board of directors will be classified into three classes of directors;

• provide that, subject to the rights of any series of preferred stock to elect directors, directors may only be removed for cause, which removal may be effected, subject to any limitation imposed by law, by the holders of at least 66 2/3% of the voting power of all of our then-outstanding shares of the capital stock entitled to vote generally at an election of directors;

• provide that all vacancies, including newly created directorships, may, except as otherwise required by law, be filled by the affirmative vote of a majority of directors then in office, even if less than a quorum;

• require that any action to be taken by our stockholders must be effected at a duly called annual or special meeting of stockholders and not be taken by written consent or electronic transmission;

• provide that stockholders seeking to present proposals before a meeting of stockholders or to nominate candidates for election as directors at a meeting of stockholders must provide advance notice in writing, and also specify requirements as to the form and content of a stockholder’s notice;

• provide that special meetings of our stockholders may be called only by the chairperson of our board of directors, our chief executive officer or by our board of directors pursuant to a resolution adopted by a majority of the total number of authorized directors; and

• not provide for cumulative voting rights, therefore allowing the holders of a majority of the shares of common stock entitled to vote in any election of directors to elect all of the directors standing for election, if they should so choose.

The amendment of any of these provisions requires approval by the holders of at least 66 2/3% of the voting power of all of our then-outstanding capital stock entitled to vote generally in the election of directors, voting together as a single class.

The combination of these provisions make it more difficult for our existing stockholders to replace our board of directors as well as for another party to obtain control of us by replacing our board of directors. Since our board of directors has the power to retain and discharge our officers, these provisions could also make it more difficult for existing stockholders or another party to effect a change in management. In addition, the authorization of undesignated preferred stock makes it possible for our board of directors to issue preferred stock with voting or other rights or preferences that could impede the success of any attempt to change our control.

These provisions are intended to enhance the likelihood of continued stability in the composition of our board of directors and its policies and to discourage coercive takeover practices and inadequate takeover bids. These provisions are also designed to reduce our vulnerability to hostile takeovers and to discourage certain tactics that may be used in proxy fights. However, such provisions could have the effect of discouraging others from making tender offers for our shares and may have the effect of delaying changes in our control or management. As a consequence, these provisions may also inhibit fluctuations in the market price of our stock.

14

Choice of Forum

Our amended and restated certificate of incorporation provides that the Court of Chancery of the State of Delaware will be the exclusive forum for: (i) any derivative action or proceeding brought on our behalf; (ii) any action asserting a breach of fiduciary duty owed by any director, officer or other employee to us or our stockholders; (iii) any action asserting a claim against us or any director or officer or other employee arising pursuant to the Delaware General Corporation Law, our amended and restated certificate of incorporation or amended and restated bylaws; or (iv) any action asserting a claim against us or any director or officer or other employee that is governed by the internal affairs doctrine. Our amended and restated certificate of incorporation further provides that the federal district courts of the United States of America will be the exclusive forum for resolving any complaint asserting a cause of action arising under the Securities Act.

The Delaware Chancery Court recently issued an opinion invalidating provisions in the certificates of incorporation of Delaware companies that purport to limit to federal court the forum in which a stockholder could bring a claim under the Securities Act. The Chancery Court held that a Delaware corporation can only use its governing documents to bind a plaintiff to a particular forum where the claim involves rights or relationships established by or under Delaware’s corporate law. Until a final resolution is reached on this matter, we will not attempt to enforce this provision of our amended and restated certificate of incorporation.

Limitations on Liability and Indemnification

Our amended and restated certificate of incorporation contains provisions that allow us to limit the liability of our current and former directors for monetary damages to the fullest extent permitted by Delaware law. Delaware law provides that directors of a corporation will not be personally liable for monetary damages for any breach of fiduciary duties as directors, except liability for:

• any breach of the director’s duty of loyalty to the corporation or its stockholders;

• any act or omission not in good faith or that involves intentional misconduct or a knowing violation of law;

• unlawful payments of dividends or unlawful stock repurchases or redemptions; or

• any transaction from which the director derived an improper personal benefit.

Such limitation of liability does not apply to liabilities arising under federal securities laws and does not affect the availability of equitable remedies such as injunctive relief or rescission.

Our amended and restated certificate of incorporation provides us with the authority to, and our amended and restated bylaws provide that we are required to, indemnify our directors and executive officers to the fullest extent permitted by Delaware law. Our amended and restated bylaws also provide that, upon satisfaction of certain conditions, we shall advance expenses incurred by a director or executive officer in advance of the final disposition of any action or proceeding, and permit us to secure insurance on behalf of any officer, director, employee, or other agent for any liability arising out of his or her actions in that capacity regardless of whether we would otherwise be permitted to indemnify him or her under the provisions of Delaware law. Our amended and restated certificate of incorporation and amended and restated bylaws also provide our board of directors with discretion to indemnify our other officers and employees when determined appropriate by our board of directors. We have entered and expect to continue to enter into agreements to indemnify our directors, executive officers and other employees as determined by the board of directors. With certain exceptions, these agreements provide for indemnification for related expenses including, among other things, attorneys’ fees, judgments, fines and settlement amounts incurred by any of these individuals in any action or proceeding.

We believe that these bylaw provisions and indemnification agreements are necessary to attract and retain qualified persons as directors and officers. We also maintain directors’ and officers’ liability insurance.

15

Exchange Listing

Our common stock is listed on The Nasdaq Global Select Market under the symbol “FTSV.”

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is American Stock Transfer & Trust Company, LLC. The transfer agent’s address is 6201 15th Avenue, Brooklyn, New York 11219, and its telephone number is(718) 921-8124.

16

DESCRIPTION OF DEBT SECURITIES

We may issue debt securities from time to time, in one or more series, as either senior or subordinated debt or as senior or subordinated convertible debt. While the terms we have summarized below will apply generally to any debt securities that we may offer under this prospectus, we will describe the particular terms of any debt securities that we may offer in more detail in the applicable prospectus supplement. The terms of any debt securities offered under a prospectus supplement may differ from the terms described below. Unless the context requires otherwise, whenever we refer to the indenture, we also are referring to any supplemental indentures that specify the terms of a particular series of debt securities.

We will issue the debt securities under the indenture that we will enter into with the trustee named in the indenture. The indenture will be qualified under the Trust Indenture Act of 1939, as amended, or the Trust Indenture Act. We have filed the form of indenture as an exhibit to the registration statement of which this prospectus is a part, and supplemental indentures and forms of debt securities containing the terms of the debt securities being offered will be filed as exhibits to the registration statement of which this prospectus is a part or will be incorporated by reference from reports that we file with the SEC.

The following summary of material provisions of the debt securities and the indenture is subject to, and qualified in its entirety by reference to, all of the provisions of the indenture applicable to a particular series of debt securities. We urge you to read the applicable prospectus supplements and any related free writing prospectuses related to the debt securities that we may offer under this prospectus, as well as the complete indenture that contains the terms of the debt securities.

General

The indenture does not limit the amount of debt securities that we may issue. It provides that we may issue debt securities up to the principal amount that we may authorize and may be in any currency or currency unit that we may designate. Except for the limitations on consolidation, merger and sale of all or substantially all of our assets contained in the indenture, the terms of the indenture do not contain any covenants or other provisions designed to give holders of any debt securities protection against changes in our operations, financial condition or transactions involving us.

We may issue the debt securities issued under the indenture as “discount securities,” which means they may be sold at a discount below their stated principal amount. These debt securities, as well as other debt securities that are not issued at a discount, may be issued with “original issue discount,” or OID, for U.S. federal income tax purposes because of interest payment and other characteristics or terms of the debt securities. Material U.S. federal income tax considerations applicable to debt securities issued with OID will be described in more detail in any applicable prospectus supplement.

We will describe in the applicable prospectus supplement the terms of the series of debt securities being offered, including:

• the title of the series of debt securities;

• any limit upon the aggregate principal amount that may be issued;

• the maturity date or dates;

• the form of the debt securities of the series;

• the applicability of any guarantees;

• whether or not the debt securities will be secured or unsecured, and the terms of any secured debt;

• whether the debt securities rank as senior debt, senior subordinated debt, subordinated debt or any combination thereof, and the terms of any subordination;

• if the price (expressed as a percentage of the aggregate principal amount thereof) at which such debt securities will be issued is a price other than the principal amount thereof, the portion of the principal

17

amount thereof payable upon declaration of acceleration of the maturity thereof, or if applicable, the portion of the principal amount of such debt securities that is convertible into another security or the method by which any such portion shall be determined;

• the interest rate or rates, which may be fixed or variable, or the method for determining the rate and the date interest will begin to accrue, the dates interest will be payable and the regular record dates for interest payment dates or the method for determining such dates;

• our right, if any, to defer payment of interest and the maximum length of any such deferral period;

• if applicable, the date or dates after which, or the period or periods during which, and the price or prices at which, we may, at our option, redeem the series of debt securities pursuant to any optional or provisional redemption provisions and the terms of those redemption provisions;

• the date or dates, if any, on which, and the price or prices at which we are obligated, pursuant to any mandatory sinking fund or analogous fund provisions or otherwise, to redeem, or at the holder’s option to purchase, the series of debt securities and the currency or currency unit in which the debt securities are payable;

• the denominations in which we will issue the series of debt securities, if other than denominations of $1,000 and any integral multiple thereof;

• any and all terms, if applicable, relating to any auction or remarketing of the debt securities of that series and any security for our obligations with respect to such debt securities and any other terms which may be advisable in connection with the marketing of debt securities of that series;

• whether the debt securities of the series shall be issued in whole or in part in the form of a global security or securities; the terms and conditions, if any, upon which such global security or securities may be exchanged in whole or in part for other individual securities; and the depositary for such global security or securities;

• if applicable, the provisions relating to conversion or exchange of any debt securities of the series and the terms and conditions upon which such debt securities will be so convertible or exchangeable, including the conversion or exchange price, as applicable, or how it will be calculated and may be adjusted, any mandatory or optional (at our option or the holders’ option) conversion or exchange features, the applicable conversion or exchange period and the manner of settlement for any conversion or exchange;

• if other than the full principal amount thereof, the portion of the principal amount of debt securities of the series which shall be payable upon declaration of acceleration of the maturity thereof;

• additions to or changes in the covenants applicable to the particular debt securities being issued, including, among others, the consolidation, merger or sale covenant;

• additions to or changes in the events of default with respect to the securities and any change in the right of the trustee or the holders to declare the principal, premium, if any, and interest, if any, with respect to such securities to be due and payable;

• additions to or changes in or deletions of the provisions relating to covenant defeasance and legal defeasance;

• additions to or changes in the provisions relating to satisfaction and discharge of the indenture;

• additions to or changes in the provisions relating to the modification of the indenture both with and without the consent of holders of debt securities issued under the indenture;

• the currency of payment of debt securities if other than U.S. dollars and the manner of determining the equivalent amount in U.S. dollars;

• whether interest will be payable in cash or additional debt securities at our or the holders’ option and the terms and conditions upon which the election may be made;

18

• the terms and conditions, if any, upon which we will pay amounts in addition to the stated interest, premium, if any and principal amounts of the debt securities of the series to any holder that is not a “United States person” for federal tax purposes;

• any restrictions on transfer, sale or assignment of the debt securities of the series; and

• any other specific terms, preferences, rights or limitations of, or restrictions on, the debt securities, any other additions or changes in the provisions of the indenture, and any terms that may be required by us or advisable under applicable laws or regulations.

Conversion or Exchange Rights

We will set forth in the applicable prospectus supplement the terms on which a series of debt securities may be convertible into or exchangeable for our common stock or our other securities. We will include provisions as to settlement upon conversion or exchange and whether conversion or exchange is mandatory, at the option of the holder or at our option. We may include provisions pursuant to which the number of shares of our common stock or our other securities that the holders of the series of debt securities receive would be subject to adjustment.

Consolidation, Merger or Sale

Unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, the indenture will not contain any covenant that restricts our ability to merge or consolidate, or sell, convey, transfer or otherwise dispose of our assets as an entirety or substantially as an entirety. However, any successor to or acquirer of such assets (other than a subsidiary of ours) must assume all of our obligations under the indenture or the debt securities, as appropriate.

Events of Default under the Indenture

Unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, the following are events of default under the indenture with respect to any series of debt securities that we may issue:

• if we fail to pay any installment of interest on any series of debt securities, as and when the same shall become due and payable, and such default continues for a period of 90 days; provided, however, that a valid extension of an interest payment period by us in accordance with the terms of any indenture supplemental thereto shall not constitute a default in the payment of interest for this purpose;

• if we fail to pay the principal of, or premium, if any, on any series of debt securities as and when the same shall become due and payable whether at maturity, upon redemption, by declaration or otherwise, or in any payment required by any sinking or analogous fund established with respect to such series; provided, however, that a valid extension of the maturity of such debt securities in accordance with the terms of any indenture supplemental thereto shall not constitute a default in the payment of principal or premium, if any;

• if we fail to observe or perform any other covenant or agreement contained in the debt securities or the indenture, other than a covenant specifically relating to another series of debt securities, and our failure continues for 90 days after we receive written notice of such failure, requiring the same to be remedied and stating that such is a notice of default thereunder, from the trustee or holders of at least 25% in aggregate principal amount of the outstanding debt securities of the applicable series; and

• if specified events of bankruptcy, insolvency or reorganization occur.

If an event of default with respect to debt securities of any series occurs and is continuing, other than an event of default specified in the last bullet point above, the trustee or the holders of at least 25% in aggregate

19

principal amount of the outstanding debt securities of that series, by notice to us in writing, and to the trustee if notice is given by such holders, may declare the unpaid principal of, premium, if any, and accrued interest, if any, due and payable immediately. If an event of default specified in the last bullet point above occurs with respect to us, the principal amount of and accrued interest, if any, of each issue of debt securities then outstanding shall be due and payable without any notice or other action on the part of the trustee or any holder.

The holders of a majority in principal amount of the outstanding debt securities of an affected series may waive any default or event of default with respect to the series and its consequences, except defaults or events of default regarding payment of principal, premium, if any, or interest, unless we have cured the default or event of default in accordance with the indenture. Any waiver shall cure the default or event of default.

Subject to the terms of the indenture, if an event of default under an indenture shall occur and be continuing, the trustee will be under no obligation to exercise any of its rights or powers under such indenture at the request or direction of any of the holders of the applicable series of debt securities, unless such holders have offered the trustee reasonable indemnity. The holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee, or exercising any trust or power conferred on the trustee, with respect to the debt securities of that series, provided that:

• the direction so given by the holder is not in conflict with any law or the applicable indenture; and

• subject to its duties under the Trust Indenture Act, the trustee need not take any action that might involve it in personal liability or might be unduly prejudicial to the holders not involved in the proceeding.

A holder of the debt securities of any series will have the right to institute a proceeding under the indenture or to appoint a receiver or trustee, or to seek other remedies only if:

• the holder has given written notice to the trustee of a continuing event of default with respect to that series;

• the holders of at least 25% in aggregate principal amount of the outstanding debt securities of that series have made written request,

• such holders have offered to the trustee indemnity satisfactory to it against the costs, expenses and liabilities to be incurred by the trustee in compliance with the request; and

• the trustee does not institute the proceeding, and does not receive from the holders of a majority in aggregate principal amount of the outstanding debt securities of that series other conflicting directions within 90 days after the notice, request and offer.

These limitations do not apply to a suit instituted by a holder of debt securities if we default in the payment of the principal, premium, if any, or interest on, the debt securities.

We will periodically file statements with the trustee regarding our compliance with specified covenants in the indenture.

Modification of Indenture; Waiver

We and the trustee may change an indenture without the consent of any holders with respect to specific matters:

• to cure any ambiguity, defect or inconsistency in the indenture or in the debt securities of any series;

• to comply with the provisions described above under “Description of Debt Securities—Consolidation, Merger or Sale;”

20

• to provide for uncertificated debt securities in addition to or in place of certificated debt securities;

• to add to our covenants, restrictions, conditions or provisions such new covenants, restrictions, conditions or provisions for the benefit of the holders of all or any series of debt securities, to make the occurrence, or the occurrence and the continuance, of a default in any such additional covenants, restrictions, conditions or provisions an event of default or to surrender any right or power conferred upon us in the indenture;

• to add to, delete from or revise the conditions, limitations, and restrictions on the authorized amount, terms, or purposes of issue, authentication and delivery of debt securities, as set forth in the indenture;

• to make any change that does not adversely affect the interests of any holder of debt securities of any series in any material respect;

• to provide for the issuance of and establish the form and terms and conditions of the debt securities of any series as provided above under “Description of Debt Securities—General” to establish the form of any certifications required to be furnished pursuant to the terms of the indenture or any series of debt securities, or to add to the rights of the holders of any series of debt securities;

• to evidence and provide for the acceptance of appointment under any indenture by a successor trustee; or

• to comply with any requirements of the SEC in connection with the qualification of any indenture under the Trust Indenture Act.

In addition, under the indenture, the rights of holders of a series of debt securities may be changed by us and the trustee with the written consent of the holders of at least a majority in aggregate principal amount of the outstanding debt securities of each series that is affected. However, unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, we and the trustee may make the following changes only with the consent of each holder of any outstanding debt securities affected:

• extending the fixed maturity of any debt securities of any series;

• reducing the principal amount, reducing the rate of or extending the time of payment of interest, or reducing any premium payable upon the redemption of any series of any debt securities; or

• reducing the percentage of debt securities, the holders of which are required to consent to any amendment, supplement, modification or waiver.

Discharge

Each indenture provides that we can elect to be discharged from our obligations with respect to one or more series of debt securities, except for specified obligations, including obligations to:

• provide for payment;

• register the transfer or exchange of debt securities of the series;

• replace stolen, lost or mutilated debt securities of the series;

• pay principal of and premium and interest on any debt securities of the series;

• maintain paying agencies;

• hold monies for payment in trust;

• recover excess money held by the trustee;

• compensate and indemnify the trustee; and

• appoint any successor trustee.

21

In order to exercise our rights to be discharged, we must deposit with the trustee money or government obligations sufficient to pay all the principal of, any premium, if any, and interest on, the debt securities of the series on the dates payments are due.

Form, Exchange and Transfer

We will issue the debt securities of each series only in fully registered form without coupons and, unless we provide otherwise in the applicable prospectus supplement, in denominations of $1,000 and any integral multiple thereof. The indenture provides that we may issue debt securities of a series in temporary or permanent global form and as book-entry securities that will be deposited with, or on behalf of, The Depository Trust Company, or DTC, or another depositary named by us and identified in the applicable prospectus supplement with respect to that series. To the extent the debt securities of a series are issued in global form and as book-entry, a description of terms relating to any book-entry securities will be set forth in the applicable prospectus supplement.

At the option of the holder, subject to the terms of the indenture and the limitations applicable to global securities described in the applicable prospectus supplement, the holder of the debt securities of any series can exchange the debt securities for other debt securities of the same series, in any authorized denomination and of like tenor and aggregate principal amount.

Subject to the terms of the indenture and the limitations applicable to global securities set forth in the applicable prospectus supplement, holders of the debt securities may present the debt securities for exchange or for registration of transfer, duly endorsed or with the form of transfer endorsed thereon duly executed if so required by us or the security registrar, at the office of the security registrar or at the office of any transfer agent designated by us for this purpose. Unless otherwise provided in the debt securities that the holder presents for transfer or exchange, we will impose no service charge for any registration of transfer or exchange, but we may require payment of any taxes or other governmental charges.

We will name in the applicable prospectus supplement the security registrar, and any transfer agent in addition to the security registrar, that we initially designate for any debt securities. We may at any time designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts, except that we will be required to maintain a transfer agent in each place of payment for the debt securities of each series.

If we elect to redeem the debt securities of any series, we will not be required to:

• issue, register the transfer of, or exchange any debt securities of that series during a period beginning at the opening of business 15 days before the day of mailing of a notice of redemption of any debt securities that may be selected for redemption and ending at the close of business on the day of the mailing; or

• register the transfer of or exchange any debt securities so selected for redemption, in whole or in part, except the unredeemed portion of any debt securities we are redeeming in part.

Information Concerning the Trustee

The trustee, other than during the occurrence and continuance of an event of default under an indenture, undertakes to perform only those duties as are specifically set forth in the applicable indenture. Upon an event of default under an indenture, the trustee must use the same degree of care as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the trustee is under no obligation to exercise any of the powers given it by the indenture at the request of any holder of debt securities unless it is offered reasonable security and indemnity against the costs, expenses and liabilities that it might incur.

22

Payment and Paying Agents

Unless we otherwise indicate in the applicable prospectus supplement, we will make payment of the interest on any debt securities on any interest payment date to the person in whose name the debt securities, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest.

We will pay principal of and any premium and interest on the debt securities of a particular series at the office of the paying agents designated by us, except that unless we otherwise indicate in the applicable prospectus supplement, we will make interest payments by check that we will mail to the holder or by wire transfer to certain holders. Unless we otherwise indicate in the applicable prospectus supplement, we will designate the corporate trust office of the trustee as our sole paying agent for payments with respect to debt securities of each series. We will name in the applicable prospectus supplement any other paying agents that we initially designate for the debt securities of a particular series. We will maintain a paying agent in each place of payment for the debt securities of a particular series.

All money we pay to a paying agent or the trustee for the payment of the principal of or any premium or interest on any debt securities that remains unclaimed at the end of two years after such principal, premium or interest has become due and payable will be repaid to us, and the holder of the debt security thereafter may look only to us for payment thereof.

Governing Law

The indenture and the debt securities will be governed by and construed in accordance with the internal laws of the State of New York, except to the extent that the Trust Indenture Act of 1939, as amended, is applicable.

23

DESCRIPTION OF WARRANTS

The following description, together with the additional information we may include in any applicable prospectus supplement and free writing prospectus, summarizes the material terms and provisions of the warrants that we may offer under this prospectus, which may consist of warrants to purchase common stock, preferred stock or debt securities and may be issued in one or more series. Warrants may be offered independently or in combination with common stock, preferred stock or debt securities offered by any prospectus supplement. While the terms we have summarized below will apply generally to any warrants that we may offer under this prospectus, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. The following description of warrants will apply to the warrants offered by this prospectus unless we provide otherwise in the applicable prospectus supplement. The applicable prospectus supplement for a particular series of warrants may specify different or additional terms.

We have filed forms of the warrant agreements and forms of warrant certificates containing the terms of the warrants that may be offered as exhibits to the registration statement of which this prospectus is a part. We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of warrant and/or the warrant agreement and warrant certificate, as applicable, that contain the terms of the particular series of warrants we are offering, and any supplemental agreements, before the issuance of such warrants. The following summaries of material terms and provisions of the warrants are subject to, and qualified in their entirety by reference to, all the provisions of the form of warrant and/or the warrant agreement and warrant certificate, as applicable, and any supplemental agreements applicable to a particular series of warrants that we may offer under this prospectus. We urge you to read the applicable prospectus supplement related to the particular series of warrants that we may offer under this prospectus, as well as any related free writing prospectus, and the complete form of warrant and/or the warrant agreement and warrant certificate, as applicable, and any supplemental agreements, that contain the terms of the warrants.

General

We will describe in the applicable prospectus supplement the terms of the series of warrants being offered, including:

• the offering price and aggregate number of warrants offered;

• the currency for which the warrants may be purchased;

• if applicable, the designation and terms of the securities with which the warrants are issued and the number of warrants issued with each such security or each principal amount of such security;

• in the case of warrants to purchase debt securities, the principal amount of debt securities purchasable upon exercise of one warrant and the price at, and currency in which, this principal amount of debt securities may be purchased upon such exercise;

• in the case of warrants to purchase common stock or preferred stock, the number of shares of common stock or preferred stock, as the case may be, purchasable upon the exercise of one warrant and the price at which these shares may be purchased upon such exercise;

• the effect of any merger, consolidation, sale or other disposition of our business on the warrant agreements and the warrants;

• the terms of any rights to redeem or call the warrants;

• any provisions for changes to or adjustments in the exercise price or number of securities issuable upon exercise of the warrants;

• the dates on which the right to exercise the warrants will commence and expire;

• the manner in which the warrant agreements and warrants may be modified;

24

• a discussion of any material or special U.S. federal income tax considerations of holding or exercising the warrants;

• the terms of the securities issuable upon exercise of the warrants; and

• any other specific terms, preferences, rights or limitations of or restrictions on the warrants.

Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including:

• in the case of warrants to purchase debt securities, the right to receive payments of principal of, or premium, if any, or interest on, the debt securities purchasable upon exercise or to enforce covenants in the applicable indenture; or

• in the case of warrants to purchase common stock or preferred stock, the right to receive dividends, if any, or, payments upon our liquidation, dissolution or winding up or to exercise voting rights, if any.

Exercise of Warrants

Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. The warrants may be exercised as set forth in the prospectus supplement relating to the warrants offered. Unless we otherwise specify in the applicable prospectus supplement, warrants may be exercised at any time up to the close of business on the expiration date set forth in the prospectus supplement relating to the warrants offered thereby. After the close of business on the expiration date, unexercised warrants will become void.

Upon receipt of payment and the warrant or warrant certificate, as applicable, properly completed and duly executed at the corporate trust office of the warrant agent, if any, or any other office, including ours, indicated in the prospectus supplement, we will, as soon as practicable, issue and deliver the securities purchasable upon such exercise. If less than all of the warrants (or the warrants represented by such warrant certificate) are exercised, a new warrant or a new warrant certificate, as applicable, will be issued for the remaining warrants.

Governing Law

Unless we otherwise specify in the applicable prospectus supplement, the warrants and any warrant agreements will be governed by and construed in accordance with the laws of the State of New York.

Enforceability of Rights by Holders of Warrants

Each warrant agent, if any, will act solely as our agent under the applicable warrant agreement and will not assume any obligation or relationship of agency or trust with any holder of any warrant. A single bank or trust company may act as warrant agent for more than one issue of warrants. A warrant agent will have no duty or responsibility in case of any default by us under the applicable warrant agreement or warrant, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a warrant may, without the consent of the related warrant agent or the holder of any other warrant, enforce by appropriate legal action its right to exercise, and receive the securities purchasable upon exercise of, its warrants.

25

LEGAL OWNERSHIP OF SECURITIES

We may issue securities in registered form or in the form of one or more global securities. We describe global securities in greater detail below. We refer to those persons who have securities registered in their own names on the books that we or any applicable trustee, depositary or warrant agent maintain for this purpose as the “holders” of those securities. These persons are the legal holders of the securities. We refer to those persons who, indirectly through others, own beneficial interests in securities that are not registered in their own names, as “indirect holders” of those securities. As we discuss below, indirect holders are not legal holders, and investors in securities issued in book-entry form or in street name will be indirect holders.

Book-Entry Holders

We may issue securities in book-entry form only, as we will specify in the applicable prospectus supplement. This means securities may be represented by one or more global securities registered in the name of a financial institution that holds them as depositary on behalf of other financial institutions that participate in the depositary’s book-entry system. These participating institutions, which are referred to as participants, in turn, hold beneficial interests in the securities on behalf of themselves or their customers.

Only the person in whose name a security is registered is recognized as the holder of that security. Securities issued in global form will be registered in the name of the depositary or its participants. Consequently, for securities issued in global form, we will recognize only the depositary as the holder of the securities, and we will make all payments on the securities to the depositary. The depositary passes along the payments it receives to its participants, which in turn pass the payments along to their customers who are the beneficial owners. The depositary and its participants do so under agreements they have made with one another or with their customers; they are not obligated to do so under the terms of the securities.

As a result, investors in a book-entry security will not own securities directly. Instead, they will own beneficial interests in a global security, through a bank, broker or other financial institution that participates in the depositary’s book-entry system or holds an interest through a participant. As long as the securities are issued in global form, investors will be indirect holders, and not holders, of the securities.

Street Name Holders

We may terminate a global security or issue securities innon-global form. In these cases, investors may choose to hold their securities in their own names or in “street name.” Securities held by an investor in street name would be registered in the name of a bank, broker or other financial institution that the investor chooses, and the investor would hold only a beneficial interest in those securities through an account he or she maintains at that institution.

For securities held in street name, we will recognize only the intermediary banks, brokers and other financial institutions in whose names the securities are registered as the holders of those securities, and we will make all payments on those securities to them. These institutions pass along the payments they receive to their customers who are the beneficial owners, but only because they agree to do so in their customer agreements or because they are legally required to do so. Investors who hold securities in street name will be indirect holders, not holders, of those securities.

Legal Holders

Our obligations, as well as the obligations of any applicable trustee and of any third parties employed by us or a trustee, run only to the legal holders of the securities. We do not have obligations to investors who hold beneficial interests in global securities, in street name or by any other indirect means. This will be the case whether an investor chooses to be an indirect holder of a security or has no choice because we are issuing the securities only in global form.

26

For example, once we make a payment or give a notice to the holder, we have no further responsibility for the payment or notice even if that holder is required, under agreements with depositary participants or customers or by law, to pass it along to the indirect holders but does not do so. Similarly, we may want to obtain the approval of the holders to amend an indenture, to relieve us of the consequences of a default or of our obligation to comply with a particular provision of the indenture or for other purposes. In such an event, we would seek approval only from the holders, and not the indirect holders, of the securities. Whether and how the holders contact the indirect holders is up to the holders.

Special Considerations For Indirect Holders

If you hold securities through a bank, broker or other financial institution, either in book-entry form or in street name, you should check with your own institution to find out:

• the performance of third-party service providers;

• how it handles securities payments and notices;

• whether it imposes fees or charges;

• how it would handle a request for the holders’ consent, if ever required;

• whether and how you can instruct it to send you securities registered in your own name so you can be a holder, if that is permitted in the future;

• how it would exercise rights under the securities if there were a default or other event triggering the need for holders to act to protect their interests; and

• if the securities are in book-entry form, how the depositary’s rules and procedures will affect these matters.

Global Securities

A global security is a security that represents one or any other number of individual securities held by a depositary. Generally, all securities represented by the same global securities will have the same terms.

Each security issued in book-entry form will be represented by a global security that we deposit with and register in the name of a financial institution or its nominee that we select. The financial institution that we select for this purpose is called the depositary. Unless we specify otherwise in the applicable prospectus supplement, DTC will be the depositary for all securities issued in book-entry form.

A global security may not be transferred to or registered in the name of anyone other than the depositary, its nominee or a successor depositary, unless special termination situations arise. We describe those situations below under the section titled “Special Situations When a Global Security Will Be Terminated” in this prospectus. As a result of these arrangements, the depositary, or its nominee, will be the sole registered owner and holder of all securities represented by a global security, and investors will be permitted to own only beneficial interests in a global security. Beneficial interests must be held by means of an account with a broker, bank or other financial institution that in turn has an account with the depositary or with another institution that does. Thus, an investor whose security is represented by a global security will not be a holder of the security, but only an indirect holder of a beneficial interest in the global security.

If the prospectus supplement for a particular security indicates that the security will be issued in global form only, then the security will be represented by a global security at all times unless and until the global security is terminated. If termination occurs, we may issue the securities through another book-entry clearing system or decide that the securities may no longer be held through any book-entry clearing system.

27

Special Considerations For Global Securities

The rights of an indirect holder relating to a global security will be governed by the account rules of the investor’s financial institution and of the depositary, as well as general laws relating to securities transfers. We do not recognize an indirect holder as a holder of securities and instead deal only with the depositary that holds the global security.

If securities are issued only in the form of a global security, an investor should be aware of the following:

• an investor cannot cause the securities to be registered in his or her name, and cannot obtainnon-global certificates for his or her interest in the securities, except in the special situations we describe below;

• an investor will be an indirect holder and must look to his or her own bank or broker for payments on the securities and protection of his or her legal rights relating to the securities, as we describe above;

• an investor may not be able to sell interests in the securities to some insurance companies and to other institutions that are required by law to own their securities innon-book-entry form;

• an investor may not be able to pledge his or her interest in a global security in circumstances where certificates representing the securities must be delivered to the lender or other beneficiary of the pledge in order for the pledge to be effective;

• the depositary’s policies, which may change from time to time, will govern payments, transfers, exchanges and other matters relating to an investor’s interest in a global security;

• we and any applicable trustee have no responsibility for any aspect of the depositary’s actions or for its records of ownership interests in a global security, nor do we or any applicable trustee supervise the depositary in any way;

• the depositary may, and we understand that DTC will, require that those who purchase and sell interests in a global security within its book-entry system use immediately available funds, and your broker or bank may require you to do so as well; and

• financial institutions that participate in the depositary’s book-entry system, and through which an investor holds its interest in a global security, may also have their own policies affecting payments, notices and other matters relating to the securities.

There may be more than one financial intermediary in the chain of ownership for an investor. We do not monitor and are not responsible for the actions of any of those intermediaries.

Special Situations When a Global Security Will Be Terminated

In a few special situations described below, the global security will terminate and interests in it will be exchanged for physical certificates representing those interests. After that exchange, the choice of whether to hold securities directly or in street name will be up to the investor. Investors must consult their own banks or brokers to find out how to have their interests in securities transferred to their own name, so that they will be direct holders. We have described the rights of holders and street name investors above.

Unless we provide otherwise in the applicable prospectus supplement, the global security will terminate when the following special situations occur:

• if the depositary notifies us that it is unwilling, unable or no longer qualified to continue as depositary for that global security and we do not appoint another institution to act as depositary within 90 days;

• if we notify any applicable trustee that we wish to terminate that global security; or

• if an event of default has occurred with regard to securities represented by that global security and has not been cured or waived.

28

The applicable prospectus supplement may also list additional situations for terminating a global security that would apply only to the particular series of securities covered by the applicable prospectus supplement. When a global security terminates, the depositary, and not we or any applicable trustee, is responsible for deciding the names of the institutions that will be the initial direct holders.

29

PLAN OF DISTRIBUTION

We may sell the securities from time to time pursuant to underwritten public offerings, direct sales to the public, “at the market” offerings, negotiated transactions, block trades or a combination of these methods. We may sell the securities to or through one or more underwriters or dealers (acting as principal or agent), through agents, or directly to one or more purchasers. We may distribute securities from time to time in one or more transactions:

• at a fixed price or prices, which may be changed;

• at market prices prevailing at the time of sale;

• at prices related to such prevailing market prices; or

• at negotiated prices.

We will describe the terms of the offering of the securities and the specific plan of distribution in a prospectus supplement or supplements to this prospectus, any related free writing prospectus that we may authorize to be provided to you, an amendment to the registration statement of which this prospectus is a part or other filings we make with the SEC under the Exchange Act that are incorporated by reference. Such description may include, to the extent applicable:

• the name or names of any underwriters, dealers, agents or other purchasers;

• the purchase price of the securities or other consideration therefor, and the proceeds, if any, we will receive from the sale;

• any options to purchase additional shares or other options under which underwriters, dealers, agents or other purchasers may purchase additional securities from us;

• any agency fees or underwriting discounts and other items constituting agents’ or underwriters’ compensation;

• any public offering price;

• any discounts or concessions allowed or reallowed or paid to dealers; and

• any securities exchange or market on which the securities may be listed.

Only underwriters named in the prospectus supplement will be underwriters of the securities offered by the prospectus supplement. Dealers and agents participating in the distribution of the securities may be deemed to be underwriters, and compensation received by them on resale of the securities may be deemed to be underwriting discounts. If such dealers or agents were deemed to be underwriters, they may be subject to statutory liabilities under the Securities Act.

If underwriters are used in the sale, they will acquire the securities for their own account and may resell the securities from time to time in one or more transactions at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. We may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Subject to certain conditions, the underwriters will be obligated to purchase all of the securities offered by the prospectus supplement, other than securities covered by any option to purchase additional shares or other option. If a dealer is used in the sale of securities, we, or an underwriter, will sell the securities to the dealer, as principal. The dealer may then resell the securities to the public at varying prices to be determined by the dealer at the time of resale. To the extent required, we will set forth in the prospectus supplement the name of the dealer and the terms of the transaction. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may change from time to time. We may use underwriters, dealers or agents with whom we have a material relationship. We will describe in the prospectus supplement, naming the underwriter, dealer or agent, the nature of any such relationship.

30

We may sell securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities and we will describe any commissions payable to the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, the agent will act on a best-efforts basis for the period of its appointment.

We may provide agents, dealers and underwriters with indemnification against civil liabilities, including liabilities under the Securities Act, or contribution with respect to payments that the agents or dealers or underwriters may make with respect to these liabilities. Agents, dealers and underwriters or their affiliates may engage in transactions with, or perform services for, us in the ordinary course of business.

All securities we may offer, other than common stock, will be new issues of securities with no established trading market. Any underwriters may make a market in these securities, but will not be obligated to do so and may discontinue any market making at any time without notice. We cannot guarantee the liquidity of the trading markets for any securities.

Any underwriter may be granted an option to purchase additional shares, and engage in stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. An underwriter’s option to purchase additional shares involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum price. Syndicate-covering or other short-covering transactions involve purchases of the securities, either through exercise of the option to purchase additional shares or in the open market after the distribution is completed, to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a stabilizing or covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

Any underwriters or dealers or agents that are qualified market makers on the Nasdaq Global Select Market may engage in passive market making transactions in the common stock on the Nasdaq Global Select Market in accordance with Regulation M under the Exchange Act, during the business day prior to the pricing of the offering, before the commencement of offers or sales of the common stock. Passive market makers must comply with applicable volume and price limitations and must be identified as passive market makers. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for such security; if all independent bids are lowered below the passive market maker’s bid, however, the passive market maker’s bid must then be lowered when certain purchase limits are exceeded. Passive market making may stabilize the market price of the securities at a level above that which might otherwise prevail in the open market and, if commenced, may be discontinued at any time.

31

LEGAL MATTERS

Unless otherwise indicated in the applicable prospectus supplement, Cooley LLP, Palo Alto, California, will pass upon the validity of the securities offered by this prospectus and any supplement thereto. As of the date of this prospectus, Cooley LLP beneficially owns 25,200 shares of our common stock.

EXPERTS

Ernst & Young LLP, independent registered public accounting firm, has audited our financial statements included in our Annual Report on Form10-K for the year ended December 31, 2018, as set forth in their report, which is incorporated by reference in this prospectus and elsewhere in the registration statement. Our financial statements are incorporated by reference in reliance on Ernst & Young LLP’s report, given on their authority as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

This prospectus is part of the registration statement on FormS-3 we filed with the SEC under the Securities Act and does not contain all the information set forth or incorporated by reference in the registration statement. Whenever a reference is made in this prospectus to any of our contracts, agreements or other documents, the reference may not be complete and you should refer to the exhibits that are a part of the registration statement or the exhibits to the reports or other documents incorporated by reference into this prospectus for a copy of such contract, agreement or other document. Because we are subject to the information and reporting requirements of the Exchange Act, we file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC’s website at http://www.sec.gov. Our Annual Report on Form10-K, Quarterly Reports on Form10-Q and Current Reports on Form8-K, including any amendments to those reports, and other information that we file with or furnish to the SEC pursuant to Section 13(a) or 15(d) of the Exchange Act can also be accessed free of charge on the Investor section of our website. These filings will be available as soon as reasonably practicable after we electronically file such material with, or furnish it to, the SEC. Our website address is http://www.fortyseveninc.com. Information contained on or accessible through our website is not a part of this prospectus and is not incorporated by reference herein, and the inclusion of our website address in this prospectus is an inactive textual reference only.

32

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to “incorporate by reference” information from other documents that we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus. Information in this prospectus supersedes information incorporated by reference that we filed with the SEC prior to the date of this prospectus, while information that we file later with the SEC will automatically update and supersede the information in this prospectus. We incorporate by reference into this prospectus and the registration statement of which this prospectus is a part the information or documents listed below that we have filed with the SEC (Commission FileNo. 001-38554):

• our Annual Report onForm 10-K for the year ended December 31, 2018, filed with the SEC on March 28, 2019;

• the information specifically incorporated by reference in our Annual Report onForm 10-K for the year ended December 31, 2018, from our Definitive Proxy Statement on Schedule  14A relating to our 2019 Annual Meeting of Stockholders, which was filed with the SEC onApril 26, 2019, as supplemented by additional materials filed with the SEC onMay 20, 2019;

• our Current Reports onForm 8-K filed with the SEC onJanuary 29, 2019,March 28, 2019 (Item 5.02),April 30, 2019,May  2, 2019,June 18, 2019 andJuly 11, 2019;

• our Quarterly Report onForm 10-Q for the three months ended March 31, 2019, filed with the SEC on May 13, 2019; and

• the description of our common stock set forth in our registration statement onForm 8-A, filed with the SEC on June 22, 2018, including any amendments thereto or reports filed for the purposes of updating this description.

All filings filed by us pursuant to the Exchange Act after the date of the initial filing of the registration statement of which this prospectus is a part effective upon filing shall be deemed to be incorporated by reference into this prospectus.

We also incorporate by reference any future filings (other than current reports furnished under Item 2.02 or Item 7.01 ofForm 8-K and exhibits filed on such form that are related to such items unless such Form8-K expressly provides to the contrary) made with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, until we file a post-effective amendment that indicates the termination of the offering of the securities made by this prospectus and will become a part of this prospectus from the date that such documents are filed with the SEC. Information in such future filings updates and supplements the information provided in this prospectus. Any statements in any such future filings will automatically be deemed to modify and supersede any information in any document we previously filed with the SEC that is incorporated or deemed to be incorporated herein by reference to the extent that statements in the later filed document modify or replace such earlier statements.

You can request a copy of these filings, at no cost, by writing or telephoning us at the following address or telephone number:

Forty Seven, Inc.

Attn: Investor Relations

1490 O’Brien Drive, Suite A

Menlo Park, California 94025

(650)352-4150

33