Cullinan Oncology (CGEM) 10-Q 14 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Entity Registrant Name	CULLINAN ONCOLOGY, INC.
Current Fiscal Year End Date	--12-31
Document Quarterly Report	true
Document Transition Report	false
Entity Central Index Key	0001789972
Entity File Number	001-39856
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-3879991
Entity Address, Address Line One	One Main Street
Entity Address, Address Line Two	Suite 520
Entity Address, City or Town	Cambridge
Entity Address, Country	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	410-4650
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	CGEM
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		43,516,125

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	[1]
Current assets:
Cash and cash equivalents	$ 293,537	$ 168,198
Prepaid expenses and other current assets	6,419	2,072
Short term investments	147,337	42,008
Total current assets	447,293	212,278
Property and equipment, net	115	130
Other assets	147	2,300
Long term investments	32,111
Total assets	479,666	214,708
Current liabilities:
Accounts payable	3,879	9,679
Accrued expenses and other current liabilities	6,015	4,641
Total current liabilities	9,894	14,320
Long-term liabilities:
Deferred rent	73	74
Total liabilities	9,967	14,394
Commitments and contingencies (Note 11)
Stockholders' equity:
Common stock, $0.0001 par value, 34,900,878 and 150,000,000 shares authorized as of December 31, 2020 and March 31, 2021, respectively; 29,381,125 and 43,516,125 shares issued and outstanding as of December 31, 2020 and March 31, 2021, respectively	4	3
Additional paid-in capital	560,366	292,348
Accumulated other comprehensive loss	(60)	(2)
Accumulated deficit	(93,409)	(93,339)
Total Cullinan stockholders' equity	466,901	199,010
Noncontrolling interests	2,798	1,304
Total stockholders' equity	469,699	200,314	[2]
Total liabilities and stockholders' equity	$ 479,666	$ 214,708
[1]	The consolidated balance sheet as of December 31, 2020 is derived from the audited consolidated financial statements as of that date and was retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.
[2]	The consolidated balance sheets as of December 31, 2019 and 2020 are derived from the audited consolidated financial statements as of those dates and were retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. Changes in stockholders’ equity for the three months ended March 31, 2020 were also retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	34,900,878
Common stock, shares issued	43,516,125	29,381,125
Common stock, shares outstanding	43,516,125	29,381,125

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Revenue	$ 18,943
Type of revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 12,415	$ 4,173
General and administrative	5,156	1,368
Total operating expenses	17,571	5,541
Income/(loss) from operations	1,372	(5,541)
Other income (expense):
Interest income	49	378
Other income (expense), net	(2)
Net Income/(loss)	1,419	(5,163)
Net income/(loss) attributable to noncontrolling interest	1,489	(190)
Net loss attributable to common stockholders of Cullinan	$ (70)	$ (4,973)
Net loss per share, basic and diluted	$ 0	$ (0.27)
Total weighted-average shares used in computing net loss per share, basic and diluted	41,977,336	18,747,004
Comprehensive income/(loss):
Net income/(loss)	$ 1,419	$ (5,163)
Unrealized gain/(loss) on investments	(58)	30
Comprehensive income/(loss)	1,361	(5,133)
Comprehensive income/(loss) attributable to noncontrolling interest	1,489	(190)
Comprehensive income/(loss) attributable to Cullinan	$ (128)	$ (4,943)

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands		Total		Common Stock	Noncontrolling Interest in Subsidiaries	Additional Paid-In Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit
Balances at Dec. 31, 2019	[1]	$ 97,865		$ 2	$ 864	$ 138,543	$ (4)	$ (41,540)
Balances, shares at Dec. 31, 2019	[1]			18,738,734
Issuance of common stock equivalents net of issuance costs of $213	[1]	14,037				14,037
Issuance of common stock equivalents net of issuance costs, shares	[1]			1,297,700
Unrealized gain/(loss) on investments		30					30
Net income/(loss)		(5,163)			(190)			(4,973)
Balances at Mar. 31, 2020		106,769		$ 2	674	152,580	26	(46,513)
Balances, shares at Mar. 31, 2020				20,036,434
Balances at Dec. 31, 2020	[1]	200,314	[2]	$ 3	1,304	292,348	(2)	(93,339)
Balances, shares at Dec. 31, 2020	[1]			29,831,125
Initial public offering net of issuance costs of $22,870		264,516		$ 1		264,515
Initial public offering net of issuance costs, shares				13,685,000
Stock based compensation		3,508			5	3,503
Unrealized gain/(loss) on investments		(58)					(58)
Net income/(loss)		1,419			1,489			(70)
Balances at Mar. 31, 2021		$ 469,699		$ 4	$ 2,798	$ 560,366	$ (60)	$ (93,409)
Balances, shares at Mar. 31, 2021				43,516,125
[1]	The consolidated balance sheets as of December 31, 2019 and 2020 are derived from the audited consolidated financial statements as of those dates and were retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. Changes in stockholders’ equity for the three months ended March 31, 2020 were also retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.
[2]	The consolidated balance sheet as of December 31, 2020 is derived from the audited consolidated financial statements as of that date and was retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.

CONSOLIDATED STATEMENTS OF ST_2

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement Of Stockholders Equity [Abstract]
Common stock equivalents, net of issuance costs		$ 213
Initial public offering, net of issuance costs	$ 22,870

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Operating activities:
Net income/(loss)		$ 1,419	$ (5,163)
Adjustments to reconcile net income/(loss) to net cash used in operating activities:
Depreciation and amortization		15	16
Equity-based compensation expense		3,508
Amortization or accretion on marketable securities		229	25
Changes in operating assets and liabilities:
Prepaid expenses and other current assets		(4,352)	693
Accounts payable		(4,124)	170
Accrued expenses and other current liabilities		1,790	(756)
Net cash used in operating activities		(1,515)	(5,015)
Investing activities:
Purchases of property and equipment			(3)
Purchase of available-for-sale securities		(149,763)	(31,087)
Proceeds from sales and maturities of investments		12,037	4,200
Net cash used in investing activities		(137,726)	(26,890)
Financing activities:
Proceeds from issuance of common stock equivalents	[1]		14,250
Proceeds from initial public offering		267,268
Payment of issuance costs related to common stock equivalents	[1]		(213)
Payment of deferred offering costs		(2,688)
Net cash provided by financing activities		264,580	14,037
Net (decrease)/increase in cash and cash equivalents		125,339	(17,868)
Cash and cash equivalents at beginning of period		168,198	63,250
Cash and cash equivalents at end of period		293,537	$ 45,382
Noncash financing activities
Deferred offering costs paid in the prior year		$ 65
[1]	Changes in equity activities for the three months ending March 31, 2020 were retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.

Nature of Business and Basis of

Nature of Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business and Basis of Presentation	(1) Nature of Business and Basis of Presentation Organization Cullinan Oncology, Inc., together with its consolidated subsidiaries (Cullinan or the Company), is a biopharmaceutical company developing a diversified pipeline of targeted oncology and immuno-oncology therapies for cancer patients. Cullinan’s predecessor company, Cullinan Pharmaceuticals, LLC was formed in September 2016 and was subsequently renamed Cullinan Oncology, LLC (the LLC) in November 2017. The LLC’s wholly-owned subsidiary, Cullinan Management, Inc. (Management), was formed in September 2016 and became the surviving entity in a reverse merger with the LLC in January 2021. In February 2021, the Company changed its name from Cullinan Management, Inc., to Cullinan Oncology, Inc. As of December 31, 2020 and March 31, 2021, the Corporation had five development subsidiaries: Cullinan Apollo Corp. (Apollo), Cullinan Pearl Corp. (Pearl), Cullinan Amber Corp. (Amber), Cullinan Florentine Corp. (Florentine) and Cullinan MICA Corp. (MICA). Reorganization, Reverse Stock Split and Initial Public Offering In January 2021, the Company completed its initial public offering (IPO) in which it issued and sold 13,685,000 shares of its common stock, including 1,785,000 shares sold pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a price to the public of $21.00 per share. The shares began trading on the Nasdaq Global Select Market on January 8, 2021 under the symbol “CGEM”. The net proceeds received by the Company from the offering were $264.5 million, after deducting underwriting discounts, commissions and other offering expenses. Immediately prior to the effectiveness of the Company’s registration statement, the Company completed its reorganization, whereby the LLC merged with and into Cullinan Management and Cullinan Management was the surviving entity (the Reorganization). Cullinan Management was the registrant in the IPO. Pursuant to the Reorganization, all outstanding Redeemable Preferred Units, Non-Voting Incentive Units and common units converted into shares of Cullinan Management common stock while common unit options were exchanged for stock options. Immediately following the Reorganization, the Company effected a one-for-7.0390 reverse stock split of its issued and outstanding shares of common stock (the Reverse Stock Split). Accordingly, all share and per share amounts for all periods presented in the accompanying unaudited consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the effect of the Reorganization and Reverse Stock Split. Refer to Note 3 for additional details relating to the Reorganization and Reverse Stock Split. Liquidity The Company has incurred operating losses and negative cash flows from operations since its inception and expects to continue to generate operating losses for the foreseeable future. The Company’s ultimate success depends on the outcome of its research and development activities as well as the ability to commercialize the Company’s product candidates. The Company is subject to a number of risks including, but not limited to, the need to obtain adequate additional funding for the ongoing and planned clinical development of its product candidates. Due to the numerous risks and uncertainties associated with pharmaceutical products and development, the Company is unable to accurately predict the timing or amount of funds required to complete development of its product candidates, and costs could exceed the Company’s expectations for a number of reasons, including reasons beyond the Company’s control. The Company incurred net losses of $5.2 million for the three months ended March 31, 2020 and recorded net income of $1.4 million for the three months ended March 31, 2021. As of March 31, 2021, the Company had an accumulated deficit of $93.4 million. Since inception, the Company has funded its operations primarily through the sale of equity securities. The Company expects that its cash, cash equivalents and short term investments as of March 31, 2021 of $440.9 million, along with long term investments of $32.1 million, will be sufficient to fund its operating expenses and capital expenditure requirements through at least the next twelve months from the date of issuance of these unaudited consolidated financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	(2) Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP) and in accordance with applicable rules and regulations of the U.S. Securities and Exchange Commission (SEC) for interim financial reporting. The unaudited financial statements include accounts of the Company and its consolidated subsidiaries. All intercompany balances have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Updates (ASUs) of the Financial Accounting Standards Board (FASB). The unaudited consolidated financial statements have been prepared on the same basis as the annual financial statements and in the opinion of the Company's management, reflect all adjustments, consisting of normal and recurring adjustments, considered necessary for a fair statement of the Company’s financial position, its results of operations and comprehensive income (loss) and its cash flows for the periods presented. The unaudited consolidated financial statements herein do not include all of the disclosures required by GAAP for a complete set of annual audited financial statements, as is permitted by such rules and regulations. The accompanying financial information should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company's Annual Report on Form 10-K filed with the SEC on March 30, 2021 for the year ended December 31, 2020 (the 2020 Form 10-K). Use of Estimates The preparation of the accompanying unaudited consolidated financial statements in accordance with GAAP requires the Company’s management to make estimates and judgments that may affect the reported amounts of assets, liabilities, revenues, expenses and related disclosure of contingent assets and liabilities. On an ongoing basis, the Company’s management evaluates the estimates, including those related to expenses and accruals. The Company’s management bases its estimates on historical experience, and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Estimates and assumptions reflected in these unaudited consolidated financial statements include but are not limited to the fair value of the royalty transfer agreements, accrued research and development costs, the valuation of acquired in process research and development asset and the fair value of equity awards issued by the Company and its subsidiaries prior to the IPO. Actual results may differ from these estimates under different assumptions or conditions. Principles of Consolidation The Company consolidates entities in which it has a controlling financial interest. The Company evaluates each of its subsidiaries to determine whether the entity represents a variable interest entity (VIE) for which consolidation should be evaluated under the VIE model, or alternatively, if the entity is a voting interest entity, for which consolidation should be evaluated using the voting interest model (VOE). The Company concluded that none of its subsidiaries is a VIE and has consolidated each subsidiary under the VOE. The Company has either created or made investments in the following entities: Consolidated Entities Relationship as of March 31, 2021 Date Control First Acquired Cullinan Apollo Corp. Partially-owned Subsidiary November 2018 Cullinan Pearl Corp. Partially-owned Subsidiary November 2018 Cullinan Amber Corp. Partially-owned Subsidiary December 2019 Cullinan Florentine Corp. Partially-owned Subsidiary December 2019 Cullinan MICA Corp. Partially-owned Subsidiary May 2020 Noncontrolling Interests To the extent that ownership interests in the Subsidiaries are held by entities other than the Company, management reports these as noncontrolling interests on the consolidated balance sheets. Earnings or losses are attributed to noncontrolling interests under the hypothetical liquidation at book value (HLBV) method. The HLBV method is a point in time calculation that utilizes inputs to determine the amount that the Company and the noncontrolling interest holders would receive upon a hypothetical liquidation at each balance sheet date based on the liquidation provisions of the respective articles of incorporation. At March 31, 2020, licensors held noncontrolling interests in Apollo and Pearl. At March 31, 2021, investors and licensors held noncontrolling interests in Apollo, Pearl, Amber, Florentine and MICA. Refer to Note 5 for details relating to the license agreements and Note 7 for details relating to the noncontrolling interests. For the three months ended March 31, 2020 and 2021, $0.2 million of net loss and $1.5 million of net income, respectively, were attributable to noncontrolling interests and recorded in the consolidated statements of operations and comprehensive income (loss) and disclosed within the noncontrolling interests in subsidiaries column in the consolidated statements of stockholders’ equity. Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. As of December 31, 2020 and March 31, 2021, cash equivalents consist of commercial paper and government-backed money market funds. Investments The Company determines the appropriate classification of its investments in debt securities at the time of purchase and re-evaluates such determination at each balance sheet date. The Company periodically reviews its investments in debt securities for impairment and adjusts these investments to their fair value when a decline in market value is deemed to be other than temporary. If losses on these securities are considered to be other than temporary, the loss is recognized in earnings. Investments not classified as cash equivalents with maturities of less than twelve months are classified as short-term available-for-sale marketable securities. Marketable securities with maturities greater than twelve months for which the Company has the intent and ability to hold the investment for greater than twelve months are classified as long-term investments in the consolidated balance sheets. Available-for-sale marketable securities are carried at estimated fair value, with unrealized gains or losses included in accumulated other comprehensive income (loss) in stockholders’ equity. The fair value of marketable securities is based on available market information. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Interest and dividends are also included in interest income. Declines in fair value judged to be other-than-temporary on available-for-sale securities, if any, are included in other income (expense), net. The Company recognized its short-term investment marketable securities by security type at December 31, 2020: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Corporate notes $ 12,780 $ — $ (3 ) $ 12,777 Commercial paper 24,473 — — 24,473 Asset-backed securities 4,757 1 — 4,758 $ 42,010 $ 1 $ (3 ) $ 42,008 The Company recognized its short-term investment marketable securities and long-term investment by security type at March 31, 2021: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Short term investments Corporate notes $ 43,468 $ 0 $ (24 ) $ 43,444 Commercial paper 94,051 2 — 94,053 Asset-backed securities 9,841 — (1 ) 9,840 Long term investments Corporate notes 32,146 — (35 ) 32,111 $ 179,506 $ 2 $ (60 ) $ 179,448 Fair Value of Financial Instruments The following table sets forth the fair value of the Company’s financial assets as of December 31, 2020, allocated into Level 1, Level 2 and Level 3, that was measured on a recurring basis: Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents Cash $ 22,007 $ — $ — $ 22,007 Money market funds 146,191 — — 146,191 Short-term investments Corporate notes — 12,777 — 12,777 Commercial paper — 24,473 — 24,473 Asset-backed securities — 4,758 — 4,758 $ 168,198 $ 42,008 $ — $ 210,206 The following table sets forth the fair value of the Company’s financial assets as of March 31, 2021, allocated into Level 1, Level 2 and Level 3, that was measured on a recurring basis: Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents Cash $ 39,021 $ — $ — $ 39,021 Money market funds 254,516 — — 254,516 Short-term investments Corporate notes — 43,444 — 43,444 Commercial paper — 94,053 — 94,053 Asset-backed securities — 9,840 — 9,840 Long-term investments Corporate notes — 32,111 — 32,111 $ 293,537 $ 179,448 $ — $ 472,985 Prepaid expenses and other current assets, accounts payable and accrued expenses and other current liabilities are carried at cost, which management believes approximates fair value due to their short term nature. Deferred Offering Costs The Company capitalizes costs that are directly associated with in-process equity financings until such financings are consummated at which time such costs are recorded against the gross proceeds of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive income (loss). The Company incurred $2.7 million of deferred offering costs and recorded such amounts against the gross proceeds of our IPO within the statements of stockholders’ equity during the three months ended March 31, 2021. Asset Acquisitions The Company measures and recognizes asset acquisitions that are not deemed to be business combinations based on the cost to acquire the assets, which includes transactions costs. Goodwill is not recognized in asset acquisitions. In an asset acquisition, the cost allocated to acquire in-process research and development (IPR&D) with no alternative future use is charged to research and development expense at the acquisition date. Revenue Recognition The Company recognizes revenue in accordance with ASC Topic 606, Revenue from Contracts with Customers, or ASC 606. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. The Company enters into licensing arrangements for research and development, manufacturing and commercialization activities, which have components within the scope of ASC 606. Payments pursuant to these arrangements typically include non-refundable, upfront payments, milestone payments upon achieving significant development events, research and development reimbursements, sales milestones and royalties on future product sales. The amount of variable consideration is constrained until it is probable that the revenue is not at a significant risk of reversal in a future period. The contracts into which the Company enters generally do not include significant financing components. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; c) the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above, and d) the measure of progress in step (v) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. Non-Exclusive Licenses If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a promise or performance obligation is distinct from the other elements, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a promise for its intended purpose without the receipt of the remaining elements, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Milestone Payments At the inception of each arrangement that includes milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license of intellectual property is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. Research and Development Costs Research and development costs are expensed as incurred and consist primarily of funds for employee wages and funds paid to third parties for the provision of services for product candidate development, clinical and preclinical development and related supply and manufacturing costs, and regulatory compliance costs. At the end of the reporting period, the Company compares payments made to third party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. Costs incurred to obtain licenses are recognized as research and development expense as incurred if the technology licensed has no alternative future use. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are received or services are performed. The Company has entered into various research and development related contracts with parties both inside and outside of the United States. The payments related to these agreements are recorded as research and development expenses as incurred. The Company records accrued liabilities for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. To date, there have been no material differences between the Company’s accrued costs and actual costs. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses in the consolidated statements of operations and comprehensive income (loss). Equity-based Compensation Equity-based compensation is measured at the grant date for all equity-based awards made to employees and non-employees based on the fair value of the awards and is recognized as expense on a straight-line basis over the requisite service period, which is generally the vesting period. The Company has elected to recognize the actual forfeitures by reducing the equity-based compensation expense in the same period as the forfeitures occur. The Company estimated the fair value of the stock options using the Black-Scholes option pricing model, which requires the input of objective and subjective assumptions. Certain assumptions used, including the fair value of the Company’s common stock prior to the time of the IPO and stock price volatility, represent management’s estimates and involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, equity-based compensation expense could be materially different for future awards. The Company classifies equity-based compensation in its consolidated statements of operations and comprehensive income (loss) in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. A reduction in the carrying value of the deferred tax assets is required when it is not more likely than not that such deferred tax assets are not realizable. Net Loss per Share Basic and diluted net loss per share is determined by dividing net loss attributable to common stockholders by the weighted-average shares of common stock outstanding during the period, without consideration for potential dilutive shares of common stock, such as stock options, unvested restricted stock awards and shares issuable under the employee stock purchase plan. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. Therefore, the weighted-average shares used to calculate both basic and diluted net loss per share are the same. The Company excluded unvested shares of restricted stock awards, stock options and shares issuable under the employee stock purchase plan from the computation of diluted weighted-average shares outstanding as they would be antidilutive. Recently Issued Accounting Pronouncements Not Yet Adopted In February 2016, the FASB issued ASU 2016-02 , Leases (Topic 842) In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Reorganization and Reverse Stoc

Reorganization and Reverse Stock Split	3 Months Ended
	Mar. 31, 2021
Reorganization And Reverse Stock Split [Abstract]
Reorganization and Reverse Stock Split	(3) Reorganization and Reverse Stock Split Immediately prior to the completion of the IPO in January 2021, the LLC completed the Reorganization. Prior to the Reorganization, Management was a wholly-owned subsidiary of the LLC. The LLC was also the direct parent company of the Company’s development subsidiaries. Pursuant to the Reorganization, Management acquired all the assets of the LLC, including all of the stock the LLC owns of Apollo, Florentine, Amber, Pearl and MICA (collectively the Asset Subsidiaries), and assumed all of the LLC’s liabilities and obligations. Following the Reorganization, all of Management’s outstanding preferred stock automatically converted on a 1-for-1 basis into common stock of Management. The Company then effected a one-for-7.0390 reverse split of its common stock (the Reverse Stock Split). Immediately prior to completion of the Company’s IPO, the existing units of the LLC were cancelled and the number of shares of common stock exchanged and issued to the LLC’s unitholders in the Reorganization is shown in the below table by unit class, on a split adjusted basis: Cullinan Oncology, LLC Unit Type Number of Management's Shares Issued Adjusted for the Reverse Stock Split Series C Preferred Units 66,599,045 9,461,414 Series B Preferred Units 63,141,016 8,970,154 Series A1 Preferred Units 50,000,000 7,103,280 Series Seed Preferred Units 16,000,000 2,273,050 Non-Voting Incentive Units 11,896,500 1,689,949 Common Units 34,747,722 4,936,415 Total shares issued 242,384,283 34,434,262 The amount presented for Non-Voting Incentive Units included unvested outstanding Non-Voting Incentive Units that were exchanged for restricted stock of Management. The amount presented for Common Units included the unvested outstanding Restricted Common Units, exchanged for Management’s restricted stock, and issued and unvested Common Unit Options, that were exchanged for Management’s stock options. The unvested awards are subject to the same time-based vesting conditions as the original awards. The Reorganization was accounted for as a reverse acquisition, where the LLC was determined to be the accounting acquirer and Management to be the legal acquiree, and recapitalization for financial reporting purposes. Accordingly, the historical financial statements of the LLC became the Company’s historical financial statements, including the comparative prior periods. All share and per share amounts in these unaudited consolidated financial statements and related notes have been retroactively adjusted, where applicable, for all periods presented. The shares of the Company’s common stock for periods prior to the Reorganization represent the outstanding units of the LLC recalculated to give effect to the Reorganization and Reverse Stock Split. All LLC units that were previously reported as Redeemable Preferred Units, or temporary equity, were converted to common stock of the Company upon the execution of the Reorganization and have been reclassified to stockholders’ equity for all periods presented, as if the Reorganization occurred at the beginning of the earliest period presented in the Company’s financial statements for the year ending December 31, 2020, as follows: As of December 31, 2019 As Reported Adjustment As Adjusted (in thousands) Redeemable Preferred Units Series Seed Redeemable Preferred Units $ 3,956 $ (3,956 ) $ — Series A1 Redeemable Preferred Units 49,946 (49,946 ) — Series B Redeemable Preferred Units 83,872 (83,872 ) — Series C Redeemable Preferred Units — — — Total Redeemable Preferred Units 137,774 (137,774 ) — Stockholders' Equity (Members' Deficit) Non-Voting Incentive Units 1 (1 ) — Common Stock — 2 2 Additional paid-in capital 770 137,773 138,543 Accumulated other comprehensive loss (4 ) — (4 ) Accumulated deficit (41,540 ) — (41,540 ) Total Cullinan Stockholders' Equity (Members' Deficit) (40,773 ) 137,774 97,001 Non-controlling interests in subsidiaries 864 — 864 Total Stockholders' Equity (Members' Deficit) $ (39,909 ) $ 137,774 $ 97,865 As of December 31, 2020 As Reported Adjustment As Adjusted (in thousands) Redeemable Preferred Units Series Seed Redeemable Preferred Units $ 3,956 $ (3,956 ) $ — Series A1 Redeemable Preferred Units 49,946 (49,946 ) — Series B Redeemable Preferred Units 97,909 (97,909 ) — Series C Redeemable Preferred Units 124,841 (124,841 ) — Total Redeemable Preferred Units 276,652 (276,652 ) — Stockholders' Equity (Members' Deficit) Non-Voting Incentive Units 1 (1 ) — Common Stock — 3 3 Additional paid-in capital 15,698 276,650 292,348 Accumulated other comprehensive loss (2 ) — (2 ) Accumulated deficit (93,339 ) — (93,339 ) Total Cullinan Stockholders' Equity (Members' Deficit) (77,642 ) 276,652 199,010 Non-controlling interests in subsidiaries 1,304 — 1,304 Total Stockholders' Equity (Members' Deficit) $ (76,338 ) $ 276,652 $ 200,314

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	3 Months Ended
	Mar. 31, 2021
Accrued Expenses And Other Current Liabilities [Abstract]
Accrued Expenses and Other Current Liabilities	(4) Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consist of the following: December 31, 2020 March 31, 2021 (in thousands) Accrued bonus $ 1,763 $ 563 Professional fees 1,018 851 Research and development costs 1,829 1,510 Other 31 91 $ 4,641 $ 3,015

License and Collaboration Agree

License and Collaboration Agreements	3 Months Ended
	Mar. 31, 2021
License And Collaboration Agreements [Abstract]
License and Collaboration Agreements	( 5 ) License and Collaboration Agreements The following table summarizes the impact of the research and development costs related to the collaboration and license agreements on the Company’s consolidated statements of operations and comprehensive (income) loss for the three months ended March 31, 2020 and 2021. For details on the structure and accounting treatment for the Company’s collaboration and license agreements, refer to the annual consolidated financial statements and notes contained in the 2020 Form 10-K. Three Months Ended March 31, 2020 2021 (in thousands) Cullinan Management - Adimab $ 104 $ — Apollo - Wistar 136 — Pearl - Taiho — 3,000 $ 240 $ 3,000 Pearl—Zai Lab License Agreement In December 2020, Pearl entered into a license agreement (Zai License Agreement) with Zai Lab Shanghai Company, Limited (Zai Lab), to grant Zai Lab, an exclusive royalty-bearing license to research, develop, commercialize and manufacture CLN-081 and products which contain CLN-081 in China, Hong Kong, Macau and Taiwan. Zai will be primarily responsible for substantially all the costs related to the operations and control over the significant activities related to the exploitation of the licensed IP. As partial consideration of the license and rights granted to Zai Lab, Zai Lab paid Pearl a one-time, irrevocable, nonrefundable license fee of $20.0 million in January 2021. In addition to the upfront fee, Zai Lab will be obligated to pay Pearl, research and development, regulatory and sales milestone payments upon the occurrence of certain milestone events in the aggregate amount of up to $211.0 million, and tiered royalty payments based on the annual net sales of the licensed product. The Company recognized the upfront license fee in revenue. The Company will also recognize milestone and royalty payments in revenue once they are earned. No milestones have been achieved to date under the Zai License Agreement. The upfront payment received by Pearl was subject to foreign tax withholdings. When the licensed IP and technology know-how was transferred to Zai Lab in January 2021, Pearl recognized revenue of $18.9 million, which represented the upfront fee less the foreign tax withholdings as Pearl does not expect to recover the amounts relating to the foreign tax withholdings. In connection with the Taiho License Agreement, Pearl is obligated to pay Taiho Pharmaceutical, Co., Ltd. (Taiho Pharma) a percentage of future revenue received from Zai Lab, including the upfront payment that was received in January 2021. Upon receipt of the upfront payment, Pearl recognized $3.0 million within research and development expenses in the consolidated statements of operations and comprehensive (income) loss.

Cullinan-MICA Asset Acquisition

Cullinan-MICA Asset Acquisition	3 Months Ended
	Mar. 31, 2021
Asset Acquisition [Abstract]
Cullinan-MICA Asset Acquisition	(6) Cullinan-MICA Asset Acquisition On May 28, 2020 (the Acquisition Date), in accordance with the Series A Senior Preferred Stock Purchase Agreement (the Purchase Agreement), the Company purchased 5,385,787 shares of Series A Senior Preferred Stock (the Series A Senior Preferred Stock) of PDI Therapeutics, Inc. (PDI Therapeutics), for $7.1 million, and certain existing PDI Therapeutics shareholders purchased approximately 702,495 shares of the Series A Senior Preferred Stock for $0.9 million. Concurrently with the Series A Senior Preferred Stock purchase, PDI Therapeutics was renamed Cullinan MICA Corp. The terms of the Purchase Agreement included two additional milestone-dependent closings for total proceeds of up to $26.0 million. Each additional closing is based on clinical development milestones of MICA’s lead candidate, CLN-619. Neither milestone occurred as of March 31, 2021. On the Acquisition Date, PDI Therapeutics authorized the issuance of 72,890,797 shares, of which 39,000,000 was designated as common stock and 33,890,797 was designated as preferred stock. Of the authorized preferred stock, PDI Therapeutics designated 1,999,998 shares as Series A Junior Preferred Stock, 652,371 shares as Series A-1 Junior Preferred Stock, 11,451,514 shares as Series A-2 Junior Preferred Stock, and 19,786,914 as Series A Senior Preferred Stock (collectively the Series Preferred Stock). Following the initial close in May 2020, MICA had 22,829,406 shares outstanding, including 6,088,282 shares of Series A Senior Preferred Stock (of which the LLC held 5,385,787 shares), 602,784 shares of common stock, and 14,103,883 shares of Series A, A-1 and A2 Junior Preferred Stock described above. In addition, there were 2,034,457 shares of common stock underlying options reserved under the 2019 Equity Incentive Plan (of which 1,826,402 was reserved for future issuances to MICA’s directors and officers, as well as former employees of PDI Therapeutics). Other than the Series A-1 Junior Preferred Stock, which shares are non-voting, the Series Preferred Stock vote equally with the shares of Common Stock. In addition to any other vote or consent, the vote or written consent of a majority of the holders of Series A Senior Preferred Stock is required for certain actions, including redemptions, dividends, distributions, dissolutions, creation of new classes of stock, mergers, sale of MICA or its assets, and amendments to the certificate of incorporation, as well as other actions. The other classes of Series Preferred Stock have voting rights pertaining to the increase or decrease in the authorized number of shares of their respective classes. The Company’s initial purchase represented approximately 24% of MICA’s fully diluted shares outstanding, including shares reserved for future issuance, and 88.5% of the Series A Senior Preferred Stock outstanding. The Company can increase its ownership to approximately 54% of MICA’s fully diluted shares outstanding by participating in the additional milestone-dependent closings. Additionally, as part of the transaction and as outlined in the Voting Agreement dated May 28, 2020, among the Company and other stockholders of MICA, MICA increased the size of its board of directors from four to five directors, of which three directors are designated by the Company. The Company also entered into a Services Agreement with MICA under which the Company will perform functions required for MICA’s operations, including accounts payable, cash management, record keeping, research and development and accounting services. Given the Company’s ownership of the Series A Senior Preferred Stock and its majority representation on MICA’s board of directors, the Company’s obtained a controlling interest in MICA on the Acquisition Date. Further, the Company evaluated the MICA transaction and determined that MICA is not a variable interest entity; however, due to the controlling interest in MICA, the Company will consolidate MICA under the voting interest model. The Company evaluated the change in control of MICA and concluded that the change in control is an asset acquisition rather than a business combination as substantially all of the value in MICA resides in CLN-619, the in-process research and development IPR&D asset developed by MICA. The cost of the assets was calculated as the sum of the fair value of the Company’s investment in MICA, the fair value of the noncontrolling interests in MICA and the Company’s transaction costs. This cost was allocated to the assets acquired and liabilities assumed in the transaction based on their relative fair values. The amount allocated to the IPR&D acquired was $6.4 million and was charged to research and development expense within the consolidated statements of operations and comprehensive income (loss) during the twelve months ended December 31, 2020 as it had no alternative future use at the time of the acquisition. In December 2020, the stockholders and the board of directors of MICA approved an amendment to its equity plan that decreased the authorized amount of shares under its equity plan such that no further grants could be made under its equity plan. Any authorized but unissued shares under the equity plan were returned to the status of authorized, unissued shares of common stock.

Common Stock and Noncontrolling

Common Stock and Noncontrolling Interest in Subsidiaries	3 Months Ended
	Mar. 31, 2021
Common Stock And Noncontrolling Interest In Subsidiaries [Abstract]
Common Stock and Noncontrolling Interest in Subsidiaries	( 7 ) Common Stock and Noncontrolling Interest in Subsidiaries Common Stock As of December 31, 2020 and March 31, 2021, 34,900,878 and 150,000,000 shares of common stock were authorized, respectively. Each share of common stock entitles the holder to one vote and to receive dividends when and if declared by the board of directors of the Company. No dividends have been declared through March 31, 2021. The LLC’s outstanding Non-Voting Incentive Units have been retroactively adjusted to reflect the effect of the Reorganization and Reverse Stock Split. Refer to Note 3 for additional details relating to the Reorganization and Reverse Stock Split. Noncontrolling Interest in Subsidiaries Certain Subsidiaries issue common stock in connection with licensing agreements and to employees, directors and consultants pursuant to subsidiary equity incentive plans. The holders of Subsidiary common stock are entitled to one vote per share. The holders of Subsidiary common stock are entitled to receive dividends when and if declared by the subsidiaries’ board of directors and distributions in either case only after the payment of all preferential amounts required to be paid to the holders of shares of Series A Preferred Stock. In November 2020, the LLC entered into a Restricted Stock Contribution Agreement (Contribution Agreement) with Amber, Pearl and Florentine where the LLC exchanged common units of the LLC for restricted common stock held by LLC employees at each of the Subsidiaries. The LLC’s outstanding restricted common units have been retroactively adjusted to reflect the effect of the Reorganization and Reverse Stock Split. Refer to Note 3 for additional details relating to the Reorganization and the Reverse Stock Split. Cullinan Apollo Corp. In December 2018, Apollo entered into a Series A Preferred Stock purchase agreement with the Company. The initial closing took place in December and Apollo sold 7,000,000 subsidiary shares of Series A Preferred Stock for gross proceeds of $7.0 million. At any time following the initial closing, upon the election of Apollo’s board of directors, Apollo may sell up to an aggregate of 11,000,000 Series A Preferred Stock shares at one or more subsequent closings at $1.00 per share. The Company did not allocate any losses to the noncontrolling interests for the three months ended March 31, 2020 and 2021. Cullinan Pearl Corp. In February 2019, Pearl entered into a Series A Preferred Stock Purchase Agreement with the Company and Taiho Ventures, LLC (Taiho Ventures) to sell up to 23,000,000 shares of Pearl’s Series A Preferred Stock for $1.00 per share. Pearl completed the initial closing of its Series A Preferred Stock where Pearl issued 14,000,000 shares of Series A Preferred Stock for $14.0 million to the Company and Taiho Ventures. In connection with the license agreement with Taiho Pharma, Pearl issued 1,860,000 shares of common stock as anti-dilution shares, in exchange for no cash consideration. In August 2020, Pearl issued 9,000,000 additional subsidiary shares of Series A Preferred Stock for $9.0 million to the Company and Taiho Ventures. In connection with the license agreement, Pearl then issued 1,206,000 additional shares of its common stock as anti-dilution shares, in exchange for no additional consideration. In November 2020, pursuant a Subscription Agreement between the Company and Pearl, the Company purchased 2,730,227 shares of Pearl’s common stock at $0.44 per share, for an aggregate purchase price of $1.2 million. Under the HLBV method, $0.2 million of losses and $1.7 million of income were attributed to noncontrolling interests for the three months ended March 31, 2020 and 2021, respectively, due to Taiho Ventures’ preferred stock ownership in Pearl. Cullinan Amber Corp. In April 2020, in connection with its Series A Preferred Stock financing, Amber issued 3,000,000 shares of its Series A Preferred Stock to the Company for gross proceeds of $3.0 million. At any time following the initial closing, upon election of Amber’s board of directors, Amber may sell up to an aggregate of 9,000,000 shares of Amber’s Series A Preferred Stock at one or more subsequent closings at $1.00 per share. In April 2020, pursuant to the license agreement between Amber and the Massachusetts Institute of Technology, Amber issued 400,132 shares of its common stock, in exchange for no additional consideration as set forth in the license agreement. The Company did not allocate any losses to the noncontrolling interests for the three months ended March 31, 2020 and 2021. Cullinan Florentine Corp. In August 2020, Florentine entered into a Series A Preferred Stock purchase agreement with the Company. The initial closing took place in August 2020 and Florentine sold 6,000,000 subsidiary shares of Series A Preferred Stock for gross proceeds of $6.0 million. At any time following the initial closing, upon the election of the Florentine’s board of directors, Florentine may sell up to an additional 6,000,000 shares of Series A Preferred Stock at one or more subsequent closings at $1.00 per share. Pursuant to the Tübingen License Agreement, Florentine issued 725,118 shares of its common stock, in exchange for no additional consideration as set forth in the license agreement. In December 2020, Florentine issued 6,000,000 additional shares of its Series A Preferred Stock for proceeds of $6,000,000 under the Florentine Series A Preferred Stock purchase agreement. Florentine issued additional 381,810 shares of its common stock to DFKZ and UFE as anti-dilution shares, in exchange for no additional consideration as set forth in the license agreement. The Company did not allocate any losses to the noncontrolling interests for the three months ended March 31, 2020 and 2021. Cullinan MICA Corp. In May 2020, MICA issued 6,088,282 million shares of Series A Senior Preferred Stock, including 5,385,787 to the Company, at $1.31 per share. See Note 6 for further details. Using a market-based approach and an option-pricing allocation method, MICA determined the fair market value of MICA’s equity at acquisition was $12.8 million, of which $7.1 million was allocated to the Company’s Series A Senior Preferred Stock position, and $5.7 million was initially allocated to noncontrolling interests, including the Junior Preferred and Common Stockholders. In December 2020, the MICA board authorized the issuance of 3,367,804 shares of MICA common stock to the Company, at a purchase price of $0.23 per share for aggregate proceeds to MICA of $0.8 million. As a result of these transactions, the Company’s ownership interests in MICA increased to 35% as of December 31, 2020. Under the HLBV method, $0.2 million of losses were attributed to noncontrolling interests for the three months ended March 31, 2021.

Equity-Based Compensation

Equity-Based Compensation	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity-Based Compensation	( 8 ) Equity-Based Compensation 2021 Stock Options and Incentive Plan The 2021 Stock Option and Incentive Plan (the 2021 Stock Plan) was adopted by the board of directors in December 2020 and the stockholders approved the plan in January 2021, prior to the completion of the Company’s IPO. The 2021 Stock Plan became effective on the day immediately prior to the effectiveness of the Company’s registration statement. The 2020 Unit Option and Grant Plan was terminated once the 2021 Stock Plan was effective. The Company initially reserved 12,546,386 shares of common stock for issuance of awards under the 2021 Stock Plan, which included (x) split-adjusted 320,228 shares of restricted stock issued under the 2021 Stock Plan in connection with the Reorganization and (y) split-adjusted equity awards comprised of (i) 4,616,187 shares of common stock issuable upon the exercise of stock options at a weighted average exercise price of $4.30; and (ii) an aggregate of 1,753,316 shares of common stock issuable upon the exercise of the stock options, with an exercise price equal to the IPO price per share, which were granted to certain executive officers, employees, and affiliates. The 2021 Stock Plan provides that the number of shares reserved and available for issuance under the 2021 Stock Plan will automatically increase each January 1, beginning on January 1, 2022, by 5% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s compensation committee. This number is subject to adjustment in the event of a stock split, stock dividend or other change in capitalization. The Company recorded equity-based compensation in the following expense categories in the consolidated statements of operations and comprehensive income (loss): Three Months Ended March 31, 2020 2021 (in thousands) Research and development $ — $ 1,569 General and administrative — 1,939 Total equity-based compensation $ — $ 3,508 2021 Employee Stock Purchase Plan The 2021 Employee Stock Purchase Plan (the ESPP) was adopted by our board of directors in December 2020 and the stockholders approved the plan in January 2021, prior to the completion of the Company’s IPO. The ESPP became effective on the day immediately prior to the effectiveness of the Company’s IPO. The Company has initially reserved 416,665 shares of common stock for issuance to participating employees. The ESPP provides that the number of shares reserved and available for issuance under the ESPP will automatically increase each January 1, beginning on January 1, 2022, by the lesser of 833,330 shares of our common stock, 1% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31, or such lesser number of shares as determined by the Company’s compensation committee. This number is subject to adjustment in the event of a stock split, stock dividend or other change in capitalization. For the Company recorded less than $0.1 million of compensation expense relating to the ESPP for the three months ended March 31, 2021.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2021
Related Party Transactions [Abstract]
Related Party Transactions	( 9 ) Related Party Transactions MPM Capital is a significant investor in the Company through one of its managed funds. In October 2016, the Company also began receiving consulting and management services pursuant to agreements with a Managing Director at MPM Capital and a principal at F2 Ventures, also a significant investor in the Company. For three months ended March 31, 2020, the Company incurred less than $0.1 million for management and advisory services in connection with those agreements. The Company did not incur any expenses relating to these management and advisory services agreements for the three months ended March 31, 2021. For the three months ended March 31, 2020 and 2021, the Company paid MPM Capital less than $0.1 million for other operational support. These expenses were recorded as general and administrative expense. In April 2020, the Company entered into a consulting agreement, or the 2020 Consulting Agreement, with Globeways Holdings Limited, or Globeways. Globeways and entities affiliated with F2 Ventures beneficially own in the aggregate greater than five percent of the Company’s outstanding units and Globeways is beneficially owned by a member of the Company’s board of directors. Pursuant to the 2020 Consulting Agreement, the board member provides leadership and advice regarding the Company’s scientific, clinical, product development and related activities and operations. Pursuant to the 2020 Consulting Agreement, the LLC pays Globeways a consulting fee at a monthly rate of $25,000. As the sole beneficial owner of Globeways, this board member receives all of the compensation paid to Globeways under the Globeways Agreements. For three months ended March 31, 2021, the Company incurred less than $0.1 million in costs related to this agreement. We do not expect to incur any further expenses as this agreement expired as of March 31, 2021. Royalty Transfer Agreements In October and December 2019 and May 2020, the Company’s subsidiaries Amber, Apollo, Florentine, MICA and Pearl entered into royalty transfer agreements with MPM Oncology Charitable Foundation, Inc. and UBS Optimus Foundation (together, the Foundations). Under these agreements, each Foundation is entitled to receive a royalty equal to 0.5% (1.0% in aggregate) of all global net sales of any products developed by the subsidiary, subject to limitations after patent expirations and on intellectual property developed after a change of control. The Company has deemed these royalty transfer agreements to be freestanding financial instruments that should be accounted for at fair value. Management has concluded that these instruments had no value at the inception of the agreements or at December 31, 2020 and March 31, 2021. As of March 31, 2021, Amber, Florentine and MICA’s programs all remained in pre-clinical research, while Apollo and Pearl’s programs were in phase 1/2a clinical trials. Given that these programs are all still in early stages of development and face inherent technical, regulatory, and competitive risks associated with achieving approval and commercialization, the Company ascribed no value to the royalty transfer agreements as of December 31, 2020 and March 31, 2021. The Company currently does not have any applicable net sales and as a result has paid no royalties under these obligation as of December 31, 2020 and March 31, 2021 nor has the Company accrued any liability as of December 31, 2020 and March 31, 2021. The Company will monitor these instruments for changes in fair value at each reporting date.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	(10) Income Taxes The Company recorded no provision for income taxes for the three months ended March 31, 2020 and 2021. Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax bases of assets and liabilities using statutory rates. Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards and research and development credits. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of federal and state deferred tax assets. Accordingly, a full valuation allowance has been established against the Company’s otherwise recognizable net deferred tax assets.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	(1 1 ) Commitments and Contingencies Operating Lease Rent expense for each of the three months ended March 31, 2020 and 2021 was $0.2 million. In December 2017, the Company signed an operating lease for 7,531 rentable square feet of office space in Cambridge, Massachusetts to commence on February 1, 2018. The lease expires on June 30, 2024. Rent expense will be recorded ratably over the lease period. The lease includes escalating rental payments, which are also being charged to rent expense ratably over the lease period. The following table summarizes future minimum payments due under the operating lease as of March 31, 2021: (in thousands) Remainder of 2021 $ 451 2022 608 2023 618 2024 313 $ 1,990

Employee Benefit Plan

Employee Benefit Plan	3 Months Ended
	Mar. 31, 2021
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plan	(1 2 ) Employee Benefit Plan The Company maintains a defined contribution 401(k) plan (the 401(k) Plan) in which employees may contribute a portion of their compensation, subject to statutory maximum contribution amounts. The Company assumes all administrative costs of the 401(k) Plan. For each of the three months ended March 31, 2020 and 2021, the expense relating to the matching contribution was less than $0.1 million.

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Net Loss per Share	(1 3 ) Net Loss per Share The following table sets forth the calculation of basic and diluted net loss per share: Three Months Ended March 31, 2020 2021 (in thousands, except share and per share data) Numerator: Net loss attributable to common stockholders of Cullinan $ (4,973 ) $ (70 ) Denominator Total weighted-average shares used in computing net loss per share, basic and diluted 18,747,004 41,977,336 Net Loss per Share: $ (0.27 ) $ (0.00 ) The following outstanding equity instruments were excluded from the computation of the diluted net loss per share for the periods presented because their effect would have been anti-dilutive: Three Months Ended March 31, 2020 2021 Restricted stock awards 478,293 242,411 Stock options — 6,600,953 Estimated shares issuable under the ESPP — 10,101 Total 478,293 6,853,465

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP) and in accordance with applicable rules and regulations of the U.S. Securities and Exchange Commission (SEC) for interim financial reporting. The unaudited financial statements include accounts of the Company and its consolidated subsidiaries. All intercompany balances have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Updates (ASUs) of the Financial Accounting Standards Board (FASB). The unaudited consolidated financial statements have been prepared on the same basis as the annual financial statements and in the opinion of the Company's management, reflect all adjustments, consisting of normal and recurring adjustments, considered necessary for a fair statement of the Company’s financial position, its results of operations and comprehensive income (loss) and its cash flows for the periods presented. The unaudited consolidated financial statements herein do not include all of the disclosures required by GAAP for a complete set of annual audited financial statements, as is permitted by such rules and regulations. The accompanying financial information should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company's Annual Report on Form 10-K filed with the SEC on March 30, 2021 for the year ended December 31, 2020 (the 2020 Form 10-K).
Use of Estimates	Use of Estimates The preparation of the accompanying unaudited consolidated financial statements in accordance with GAAP requires the Company’s management to make estimates and judgments that may affect the reported amounts of assets, liabilities, revenues, expenses and related disclosure of contingent assets and liabilities. On an ongoing basis, the Company’s management evaluates the estimates, including those related to expenses and accruals. The Company’s management bases its estimates on historical experience, and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Estimates and assumptions reflected in these unaudited consolidated financial statements include but are not limited to the fair value of the royalty transfer agreements, accrued research and development costs, the valuation of acquired in process research and development asset and the fair value of equity awards issued by the Company and its subsidiaries prior to the IPO. Actual results may differ from these estimates under different assumptions or conditions.
Principles of Consolidation	Principles of Consolidation The Company consolidates entities in which it has a controlling financial interest. The Company evaluates each of its subsidiaries to determine whether the entity represents a variable interest entity (VIE) for which consolidation should be evaluated under the VIE model, or alternatively, if the entity is a voting interest entity, for which consolidation should be evaluated using the voting interest model (VOE). The Company concluded that none of its subsidiaries is a VIE and has consolidated each subsidiary under the VOE. The Company has either created or made investments in the following entities: Consolidated Entities Relationship as of March 31, 2021 Date Control First Acquired Cullinan Apollo Corp. Partially-owned Subsidiary November 2018 Cullinan Pearl Corp. Partially-owned Subsidiary November 2018 Cullinan Amber Corp. Partially-owned Subsidiary December 2019 Cullinan Florentine Corp. Partially-owned Subsidiary December 2019 Cullinan MICA Corp. Partially-owned Subsidiary May 2020
Noncontrolling Interests	Noncontrolling Interests To the extent that ownership interests in the Subsidiaries are held by entities other than the Company, management reports these as noncontrolling interests on the consolidated balance sheets. Earnings or losses are attributed to noncontrolling interests under the hypothetical liquidation at book value (HLBV) method. The HLBV method is a point in time calculation that utilizes inputs to determine the amount that the Company and the noncontrolling interest holders would receive upon a hypothetical liquidation at each balance sheet date based on the liquidation provisions of the respective articles of incorporation. At March 31, 2020, licensors held noncontrolling interests in Apollo and Pearl. At March 31, 2021, investors and licensors held noncontrolling interests in Apollo, Pearl, Amber, Florentine and MICA. Refer to Note 5 for details relating to the license agreements and Note 7 for details relating to the noncontrolling interests. For the three months ended March 31, 2020 and 2021, $0.2 million of net loss and $1.5 million of net income, respectively, were attributable to noncontrolling interests and recorded in the consolidated statements of operations and comprehensive income (loss) and disclosed within the noncontrolling interests in subsidiaries column in the consolidated statements of stockholders’ equity.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. As of December 31, 2020 and March 31, 2021, cash equivalents consist of commercial paper and government-backed money market funds.
Investments	Investments The Company determines the appropriate classification of its investments in debt securities at the time of purchase and re-evaluates such determination at each balance sheet date. The Company periodically reviews its investments in debt securities for impairment and adjusts these investments to their fair value when a decline in market value is deemed to be other than temporary. If losses on these securities are considered to be other than temporary, the loss is recognized in earnings. Investments not classified as cash equivalents with maturities of less than twelve months are classified as short-term available-for-sale marketable securities. Marketable securities with maturities greater than twelve months for which the Company has the intent and ability to hold the investment for greater than twelve months are classified as long-term investments in the consolidated balance sheets. Available-for-sale marketable securities are carried at estimated fair value, with unrealized gains or losses included in accumulated other comprehensive income (loss) in stockholders’ equity. The fair value of marketable securities is based on available market information. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest income. Interest and dividends are also included in interest income. Declines in fair value judged to be other-than-temporary on available-for-sale securities, if any, are included in other income (expense), net. The Company recognized its short-term investment marketable securities by security type at December 31, 2020: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Corporate notes $ 12,780 $ — $ (3 ) $ 12,777 Commercial paper 24,473 — — 24,473 Asset-backed securities 4,757 1 — 4,758 $ 42,010 $ 1 $ (3 ) $ 42,008 The Company recognized its short-term investment marketable securities and long-term investment by security type at March 31, 2021: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Short term investments Corporate notes $ 43,468 $ 0 $ (24 ) $ 43,444 Commercial paper 94,051 2 — 94,053 Asset-backed securities 9,841 — (1 ) 9,840 Long term investments Corporate notes 32,146 — (35 ) 32,111 $ 179,506 $ 2 $ (60 ) $ 179,448
Fair Value of Financial Instruments	Fair Value of Financial Instruments The following table sets forth the fair value of the Company’s financial assets as of December 31, 2020, allocated into Level 1, Level 2 and Level 3, that was measured on a recurring basis: Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents Cash $ 22,007 $ — $ — $ 22,007 Money market funds 146,191 — — 146,191 Short-term investments Corporate notes — 12,777 — 12,777 Commercial paper — 24,473 — 24,473 Asset-backed securities — 4,758 — 4,758 $ 168,198 $ 42,008 $ — $ 210,206 The following table sets forth the fair value of the Company’s financial assets as of March 31, 2021, allocated into Level 1, Level 2 and Level 3, that was measured on a recurring basis: Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents Cash $ 39,021 $ — $ — $ 39,021 Money market funds 254,516 — — 254,516 Short-term investments Corporate notes — 43,444 — 43,444 Commercial paper — 94,053 — 94,053 Asset-backed securities — 9,840 — 9,840 Long-term investments Corporate notes — 32,111 — 32,111 $ 293,537 $ 179,448 $ — $ 472,985 Prepaid expenses and other current assets, accounts payable and accrued expenses and other current liabilities are carried at cost, which management believes approximates fair value due to their short term nature.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes costs that are directly associated with in-process equity financings until such financings are consummated at which time such costs are recorded against the gross proceeds of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive income (loss). The Company incurred $2.7 million of deferred offering costs and recorded such amounts against the gross proceeds of our IPO within the statements of stockholders’ equity during the three months ended March 31, 2021.
Asset Acquisitions	Asset Acquisitions The Company measures and recognizes asset acquisitions that are not deemed to be business combinations based on the cost to acquire the assets, which includes transactions costs. Goodwill is not recognized in asset acquisitions. In an asset acquisition, the cost allocated to acquire in-process research and development (IPR&D) with no alternative future use is charged to research and development expense at the acquisition date.
Revenue Recognition	Revenue Recognition The Company recognizes revenue in accordance with ASC Topic 606, Revenue from Contracts with Customers, or ASC 606. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. The Company enters into licensing arrangements for research and development, manufacturing and commercialization activities, which have components within the scope of ASC 606. Payments pursuant to these arrangements typically include non-refundable, upfront payments, milestone payments upon achieving significant development events, research and development reimbursements, sales milestones and royalties on future product sales. The amount of variable consideration is constrained until it is probable that the revenue is not at a significant risk of reversal in a future period. The contracts into which the Company enters generally do not include significant financing components. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; c) the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above, and d) the measure of progress in step (v) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. Non-Exclusive Licenses If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a promise or performance obligation is distinct from the other elements, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a promise for its intended purpose without the receipt of the remaining elements, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Milestone Payments At the inception of each arrangement that includes milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license of intellectual property is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred and consist primarily of funds for employee wages and funds paid to third parties for the provision of services for product candidate development, clinical and preclinical development and related supply and manufacturing costs, and regulatory compliance costs. At the end of the reporting period, the Company compares payments made to third party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. Costs incurred to obtain licenses are recognized as research and development expense as incurred if the technology licensed has no alternative future use. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are received or services are performed. The Company has entered into various research and development related contracts with parties both inside and outside of the United States. The payments related to these agreements are recorded as research and development expenses as incurred. The Company records accrued liabilities for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. To date, there have been no material differences between the Company’s accrued costs and actual costs.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses in the consolidated statements of operations and comprehensive income (loss).
Equity-based Compensation	Equity-based Compensation Equity-based compensation is measured at the grant date for all equity-based awards made to employees and non-employees based on the fair value of the awards and is recognized as expense on a straight-line basis over the requisite service period, which is generally the vesting period. The Company has elected to recognize the actual forfeitures by reducing the equity-based compensation expense in the same period as the forfeitures occur. The Company estimated the fair value of the stock options using the Black-Scholes option pricing model, which requires the input of objective and subjective assumptions. Certain assumptions used, including the fair value of the Company’s common stock prior to the time of the IPO and stock price volatility, represent management’s estimates and involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, equity-based compensation expense could be materially different for future awards. The Company classifies equity-based compensation in its consolidated statements of operations and comprehensive income (loss) in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. A reduction in the carrying value of the deferred tax assets is required when it is not more likely than not that such deferred tax assets are not realizable.
Net Loss per Share	Net Loss per Share Basic and diluted net loss per share is determined by dividing net loss attributable to common stockholders by the weighted-average shares of common stock outstanding during the period, without consideration for potential dilutive shares of common stock, such as stock options, unvested restricted stock awards and shares issuable under the employee stock purchase plan. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. Therefore, the weighted-average shares used to calculate both basic and diluted net loss per share are the same. The Company excluded unvested shares of restricted stock awards, stock options and shares issuable under the employee stock purchase plan from the computation of diluted weighted-average shares outstanding as they would be antidilutive.
Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Issued Accounting Pronouncements Not Yet Adopted In February 2016, the FASB issued ASU 2016-02 , Leases (Topic 842) In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Schedule of Short-term and Long-term Investment Marketable Securities by Security Type	The Company recognized its short-term investment marketable securities by security type at December 31, 2020: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Corporate notes $ 12,780 $ — $ (3 ) $ 12,777 Commercial paper 24,473 — — 24,473 Asset-backed securities 4,757 1 — 4,758 $ 42,010 $ 1 $ (3 ) $ 42,008 The Company recognized its short-term investment marketable securities and long-term investment by security type at March 31, 2021: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Short term investments Corporate notes $ 43,468 $ 0 $ (24 ) $ 43,444 Commercial paper 94,051 2 — 94,053 Asset-backed securities 9,841 — (1 ) 9,840 Long term investments Corporate notes 32,146 — (35 ) 32,111 $ 179,506 $ 2 $ (60 ) $ 179,448
Summary of Fair Value of Financial Assets Measured on Recurring Basis	The following table sets forth the fair value of the Company’s financial assets as of December 31, 2020, allocated into Level 1, Level 2 and Level 3, that was measured on a recurring basis: Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents Cash $ 22,007 $ — $ — $ 22,007 Money market funds 146,191 — — 146,191 Short-term investments Corporate notes — 12,777 — 12,777 Commercial paper — 24,473 — 24,473 Asset-backed securities — 4,758 — 4,758 $ 168,198 $ 42,008 $ — $ 210,206 The following table sets forth the fair value of the Company’s financial assets as of March 31, 2021, allocated into Level 1, Level 2 and Level 3, that was measured on a recurring basis: Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents Cash $ 39,021 $ — $ — $ 39,021 Money market funds 254,516 — — 254,516 Short-term investments Corporate notes — 43,444 — 43,444 Commercial paper — 94,053 — 94,053 Asset-backed securities — 9,840 — 9,840 Long-term investments Corporate notes — 32,111 — 32,111 $ 293,537 $ 179,448 $ — $ 472,985

Reorganization and Reverse St_2

Reorganization and Reverse Stock Split (Tables)	3 Months Ended
	Mar. 31, 2021
Reorganization And Reverse Stock Split [Abstract]
Number of Shares of Common Stock Exchanged and Issued to Unitholders on Split Adjusted Basis	the existing units of the LLC were cancelled and the number of shares of common stock exchanged and issued to the LLC’s unitholders in the Reorganization is shown in the below table by unit class, on a split adjusted basis: Cullinan Oncology, LLC Unit Type Number of Management's Shares Issued Adjusted for the Reverse Stock Split Series C Preferred Units 66,599,045 9,461,414 Series B Preferred Units 63,141,016 8,970,154 Series A1 Preferred Units 50,000,000 7,103,280 Series Seed Preferred Units 16,000,000 2,273,050 Non-Voting Incentive Units 11,896,500 1,689,949 Common Units 34,747,722 4,936,415 Total shares issued 242,384,283 34,434,262
Summary of LLC Units Previously Reported as Redeemable Preferred Units or Temporary Equity Converted Reclassified to Equity	All LLC units that were previously reported as Redeemable Preferred Units, or temporary equity, were converted to common stock of the Company upon the execution of the Reorganization and have been reclassified to stockholders’ equity for all periods presented, as if the Reorganization occurred at the beginning of the earliest period presented in the Company’s financial statements for the year ending December 31, 2020, as follows: As of December 31, 2019 As Reported Adjustment As Adjusted (in thousands) Redeemable Preferred Units Series Seed Redeemable Preferred Units $ 3,956 $ (3,956 ) $ — Series A1 Redeemable Preferred Units 49,946 (49,946 ) — Series B Redeemable Preferred Units 83,872 (83,872 ) — Series C Redeemable Preferred Units — — — Total Redeemable Preferred Units 137,774 (137,774 ) — Stockholders' Equity (Members' Deficit) Non-Voting Incentive Units 1 (1 ) — Common Stock — 2 2 Additional paid-in capital 770 137,773 138,543 Accumulated other comprehensive loss (4 ) — (4 ) Accumulated deficit (41,540 ) — (41,540 ) Total Cullinan Stockholders' Equity (Members' Deficit) (40,773 ) 137,774 97,001 Non-controlling interests in subsidiaries 864 — 864 Total Stockholders' Equity (Members' Deficit) $ (39,909 ) $ 137,774 $ 97,865 As of December 31, 2020 As Reported Adjustment As Adjusted (in thousands) Redeemable Preferred Units Series Seed Redeemable Preferred Units $ 3,956 $ (3,956 ) $ — Series A1 Redeemable Preferred Units 49,946 (49,946 ) — Series B Redeemable Preferred Units 97,909 (97,909 ) — Series C Redeemable Preferred Units 124,841 (124,841 ) — Total Redeemable Preferred Units 276,652 (276,652 ) — Stockholders' Equity (Members' Deficit) Non-Voting Incentive Units 1 (1 ) — Common Stock — 3 3 Additional paid-in capital 15,698 276,650 292,348 Accumulated other comprehensive loss (2 ) — (2 ) Accumulated deficit (93,339 ) — (93,339 ) Total Cullinan Stockholders' Equity (Members' Deficit) (77,642 ) 276,652 199,010 Non-controlling interests in subsidiaries 1,304 — 1,304 Total Stockholders' Equity (Members' Deficit) $ (76,338 ) $ 276,652 $ 200,314

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	3 Months Ended
	Mar. 31, 2021
Accrued Expenses And Other Current Liabilities [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following: December 31, 2020 March 31, 2021 (in thousands) Accrued bonus $ 1,763 $ 563 Professional fees 1,018 851 Research and development costs 1,829 1,510 Other 31 91 $ 4,641 $ 3,015

License and Collaboration Agr_2

License and Collaboration Agreements (Tables)	3 Months Ended
	Mar. 31, 2021
License And Collaboration Agreements [Abstract]
Summary of Research and Development Costs Related to Collaboration and License Agreements	The following table summarizes the impact of the research and development costs related to the collaboration and license agreements on the Company’s consolidated statements of operations and comprehensive (income) loss for the three months ended March 31, 2020 and 2021. For details on the structure and accounting treatment for the Company’s collaboration and license agreements, refer to the annual consolidated financial statements and notes contained in the 2020 Form 10-K. Three Months Ended March 31, 2020 2021 (in thousands) Cullinan Management - Adimab $ 104 $ — Apollo - Wistar 136 — Pearl - Taiho — 3,000 $ 240 $ 3,000

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Equity-based Compensation Expense Categories In Consolidated Statements of Operations and Comprehensive Income (Loss)	The Company recorded equity-based compensation in the following expense categories in the consolidated statements of operations and comprehensive income (loss): Three Months Ended March 31, 2020 2021 (in thousands) Research and development $ — $ 1,569 General and administrative — 1,939 Total equity-based compensation $ — $ 3,508

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Payments Due Under Operating Lease	The following table summarizes future minimum payments due under the operating lease as of March 31, 2021: (in thousands) Remainder of 2021 $ 451 2022 608 2023 618 2024 313 $ 1,990

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Summary of Calculation of Basic and Diluted Net Loss Per Share	The following table sets forth the calculation of basic and diluted net loss per share: Three Months Ended March 31, 2020 2021 (in thousands, except share and per share data) Numerator: Net loss attributable to common stockholders of Cullinan $ (4,973 ) $ (70 ) Denominator Total weighted-average shares used in computing net loss per share, basic and diluted 18,747,004 41,977,336 Net Loss per Share: $ (0.27 ) $ (0.00 )
Summary of Outstanding Equity Instruments Excluded from Computation of Diluted Net Loss Per Share	The following outstanding equity instruments were excluded from the computation of the diluted net loss per share for the periods presented because their effect would have been anti-dilutive: Three Months Ended March 31, 2020 2021 Restricted stock awards 478,293 242,411 Stock options — 6,600,953 Estimated shares issuable under the ESPP — 10,101 Total 478,293 6,853,465

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2021USD ($)$ / sharesshares	Mar. 31, 2021USD ($)Subsidiary	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)Subsidiary		Dec. 31, 2019USD ($)
Nature Of Business And Basis Of Presentation [Line Items]
Number of development subsidiaries | Subsidiary		5		5
Proceeds from initial public offering	$ 264,500	$ 267,268
Reverse stock split conversion ratio		1
Reverse stock split		one-for-7.0390
Net Income (loss)		$ 1,419	$ (5,163)
Accumulated deficit		(93,409)		$ (93,339)	[1]	$ (41,540)
Short term investments		440,900
Long term investments		$ 32,111
IPO
Nature Of Business And Basis Of Presentation [Line Items]
Common stock shares issued and sold | shares	13,685,000
Stock, price per Share | $ / shares	$ 21
Underwriters’ Option
Nature Of Business And Basis Of Presentation [Line Items]
Common stock shares issued and sold | shares	1,785,000
[1]	The consolidated balance sheet as of December 31, 2020 is derived from the audited consolidated financial statements as of that date and was retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounting Policies [Abstract]
Net (income) loss attributable to noncontrolling interests	$ (1.5)	$ 0.2
Deferred offering costs	$ 2.7

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Short-term and Long-term Investment Marketable Securities by Security Type (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	$ 179,506	$ 42,010
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(60)	(3)
Estimated Fair Value	179,448	42,008
Corporate Note Securities
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	43,468	12,780
Gross Unrealized Gains	0
Gross Unrealized Losses	(24)	(3)
Estimated Fair Value	43,444	12,777
Corporate Note Securities | Long Term Investments
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	32,146
Gross Unrealized Losses	(35)
Estimated Fair Value	32,111
Asset-backed Securities
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	9,841	4,757
Gross Unrealized Gains		1
Gross Unrealized Losses	(1)
Estimated Fair Value	9,840	4,758
Commercial Paper
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	94,051	24,473
Gross Unrealized Gains	2
Estimated Fair Value	$ 94,053	$ 24,473

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Fair Value of Financial Assets Measured on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 472,985	$ 210,206
Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	39,021	22,007
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	254,516	146,191
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	293,537	168,198
Level 1 | Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	39,021	22,007
Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	254,516	146,191
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	179,448	42,008
Corporate Note Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments	43,444	12,777
Long-term investments	32,111
Corporate Note Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments	43,444	12,777
Long-term investments	32,111
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments	94,053	24,473
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments	94,053	24,473
Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments	9,840	4,758
Asset-backed Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short term investments	$ 9,840	$ 4,758

Reorganization and Reverse St_3

Reorganization and Reverse Stock Split - Additional Information (Details)	3 Months Ended
	Mar. 31, 2021
Reorganization And Reverse Stock Split [Abstract]
Reverse stock split conversion ratio	1
Reverse stock split	one-for-7.0390

Reorganization and Reverse St_4

Reorganization and Reverse Stock Split - Number of Shares of Common Stock Exchanged and Issued to Unitholders on Split Adjusted Basis (Details) - Cullinan Oncology, LLC	3 Months Ended
	Mar. 31, 2021shares
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	242,384,283
Adjusted for the Reverse Stock Split	34,434,262
Series C Preferred Units
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	66,599,045
Adjusted for the Reverse Stock Split	9,461,414
Series B Preferred Units
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	63,141,016
Adjusted for the Reverse Stock Split	8,970,154
Series A1 Preferred Units
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	50,000,000
Adjusted for the Reverse Stock Split	7,103,280
Series Seed Preferred Units
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	16,000,000
Adjusted for the Reverse Stock Split	2,273,050
Non-Voting Incentive Units
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	11,896,500
Adjusted for the Reverse Stock Split	1,689,949
Common Units
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	34,747,722
Adjusted for the Reverse Stock Split	4,936,415

Reorganization and Reverse St_5

Reorganization and Reverse Stock Split - Summary of LLC Units Previously Reported as Redeemable Preferred Units or Temporary Equity Converted Reclassified to Equity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020		Mar. 31, 2020	Dec. 31, 2019
Stockholders' Equity (Members' Deficit)
Common Stock	$ 4	$ 3	[1]		$ 2
Additional paid-in capital	560,366	292,348	[1]		138,543
Accumulated other comprehensive loss	(60)	(2)	[1]		(4)
Accumulated deficit	(93,409)	(93,339)	[1]		(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)	466,901	199,010	[1]		97,001
Non-controlling interests in subsidiaries	2,798	1,304	[1]		864
Total Stockholders' Equity (Members' Deficit)	$ 469,699	200,314	[1],[2]	$ 106,769	97,865	[2]
As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		276,652			137,774
Stockholders' Equity (Members' Deficit)
Non-Voting Incentive Units		1			1
Additional paid-in capital		15,698			770
Accumulated other comprehensive loss		(2)			(4)
Accumulated deficit		(93,339)			(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)		(77,642)			(40,773)
Non-controlling interests in subsidiaries		1,304			864
Total Stockholders' Equity (Members' Deficit)		(76,338)			(39,909)
Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(276,652)			(137,774)
Stockholders' Equity (Members' Deficit)
Non-Voting Incentive Units		(1)			(1)
Common Stock		3			2
Additional paid-in capital		276,650			137,773
Total Cullinan Stockholders' Equity (Members' Deficit)		276,652			137,774
Total Stockholders' Equity (Members' Deficit)		276,652			137,774
Series Seed Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		3,956			3,956
Series Seed Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(3,956)			(3,956)
Series A1 Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		49,946			49,946
Series A1 Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(49,946)			(49,946)
Series B Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		97,909			83,872
Series B Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(97,909)			$ (83,872)
Series C Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		124,841
Series C Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		$ (124,841)
[1]	The consolidated balance sheet as of December 31, 2020 is derived from the audited consolidated financial statements as of that date and was retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.
[2]	The consolidated balance sheets as of December 31, 2019 and 2020 are derived from the audited consolidated financial statements as of those dates and were retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. Changes in stockholders’ equity for the three months ended March 31, 2020 were also retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accounts Payable And Accrued Liabilities Current [Abstract]
Accrued bonus	$ 563	$ 1,763
Professional fees	851	1,018
Research and development costs	1,510	1,829
Other	91	31
Accrued expenses and other current liabilities	$ 3,015	$ 4,641

License and Collaboration Agr_3

License and Collaboration Agreements - Summary of Research and Development Costs Related to Collaboration and License Agreements (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2021	Mar. 31, 2021	Mar. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs		$ 12,415	$ 4,173
Collaboration and License Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs		3,000	240
Collaboration and License Agreements | Cullinan Management - Adimab
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs			104
Collaboration and License Agreements | Apollo - Wistar
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs			$ 136
Collaboration and License Agreements | Pearl - Taiho
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs	$ 3,000	$ 3,000

License and Collaboration Agr_4

License and Collaboration Agreements - Additional Information (Details) - USD ($)	Dec. 31, 2020	Jan. 31, 2021	Mar. 31, 2021	Mar. 31, 2020
License And Collaboration Agreements [Line Items]
Net upfront revenue			$ 18,943,000
Type of revenue [Extensible List]			us-gaap:LicenseMember	us-gaap:LicenseMember
Research and development costs			$ 12,415,000	$ 4,173,000
Collaboration and License Agreements
License And Collaboration Agreements [Line Items]
Research and development costs			3,000,000	$ 240,000
Pearl—Zai Lab License Agreement
License And Collaboration Agreements [Line Items]
Nonrefundable license fee		$ 20,000,000
Net upfront revenue		$ 18,900,000
Type of revenue [Extensible List]		us-gaap:LicenseMember
Pearl—Zai Lab License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment	$ 211,000,000
Pearl - Taiho | Collaboration and License Agreements
License And Collaboration Agreements [Line Items]
Research and development costs		$ 3,000,000	$ 3,000,000

Cullinan-MICA Asset Acquisiti_2

Cullinan-MICA Asset Acquisition - Additional Information (Details)	May 28, 2020USD ($)shares	Mar. 31, 2021USD ($)Milestoneshares	Dec. 31, 2020USD ($)shares	May 31, 2020shares
Schedule Of Asset Acquisition [Line Items]
Common stock, shares authorized		150,000,000	34,900,878
Common stock, shares outstanding		43,516,125	29,381,125
Voting agreement description		Voting Agreement dated May 28, 2020, among the Company and other stockholders of MICA, MICA increased the size of its board of directors from four to five directors, of which three directors are designated by the Company.
PDI Therapeutics, Inc.
Schedule Of Asset Acquisition [Line Items]
Number of share authorized for issuance	72,890,797
Common stock, shares authorized	39,000,000
Preferred stock shares authorized	33,890,797
MICA
Schedule Of Asset Acquisition [Line Items]
Number of shares outstanding				22,829,406
Common stock, shares outstanding				602,784
2019 Equity Incentive Plan
Schedule Of Asset Acquisition [Line Items]
Common stock, shares reserved				2,034,457
MICAs Directors and Officers, as well as Former Employees of PDI Therapeutics | 2019 Equity Incentive Plan
Schedule Of Asset Acquisition [Line Items]
Common stock, shares reserved				1,826,402
Series A Senior Preferred Stock | PDI Therapeutics, Inc.
Schedule Of Asset Acquisition [Line Items]
Preferred stock shares authorized	19,786,914
Series A Senior Preferred Stock | MICA
Schedule Of Asset Acquisition [Line Items]
Ownership percentage in outstanding shares		88.50%
Series A Junior Preferred Stock | PDI Therapeutics, Inc.
Schedule Of Asset Acquisition [Line Items]
Preferred stock shares authorized	1,999,998
Series A Junior Preferred Stock | MICA
Schedule Of Asset Acquisition [Line Items]
Preferred stock shares outstanding				6,088,282
Series A Junior Preferred Stock | Cullinan Oncology, LLC
Schedule Of Asset Acquisition [Line Items]
Preferred stock shares outstanding				5,385,787
Series A-1 Junior Preferred Stock | PDI Therapeutics, Inc.
Schedule Of Asset Acquisition [Line Items]
Preferred stock shares authorized	652,371
Series A-2 Junior Preferred Stock | PDI Therapeutics, Inc.
Schedule Of Asset Acquisition [Line Items]
Preferred stock shares authorized	11,451,514
Series A, A-1 and A2 Junior Preferred Stock | MICA
Schedule Of Asset Acquisition [Line Items]
Preferred stock shares outstanding				14,103,883
Fully Diluted Shares | MICA
Schedule Of Asset Acquisition [Line Items]
Ownership percentage in outstanding shares		24.00%
Ownership percentage can increase by participating in additional milestone-dependent closings		54.00%
Research and Development Expense
Schedule Of Asset Acquisition [Line Items]
In-process research and development acquired | $			$ 6,400,000
Series A Senior Preferred Stock Purchase Agreement
Schedule Of Asset Acquisition [Line Items]
Number of additional milestone | Milestone		2
Series A Senior Preferred Stock Purchase Agreement | Series A Senior Preferred Stock | PDI Therapeutics, Inc.
Schedule Of Asset Acquisition [Line Items]
Number of share purchased as part of asset acquisition	5,385,787
Share purchased price as part of asset acquisition | $	$ 7,100,000
Series A Senior Preferred Stock Purchase Agreement | Series A Senior Preferred Stock | PDI Therapeutics, Inc. | Certain Existing PDI Therapeutics Shareholders
Schedule Of Asset Acquisition [Line Items]
Number of share purchased as part of asset acquisition	702,495
Share purchased price as part of asset acquisition | $	$ 900,000
Series A Senior Preferred Stock Purchase Agreement | Maximum
Schedule Of Asset Acquisition [Line Items]
Proceeds from preferred stock | $		$ 26,000,000

Common Stock and Noncontrolli_2

Common Stock and Noncontrolling Interest in Subsidiaries - Additional Information (Details) - USD ($)		May 28, 2020	Dec. 31, 2020	Nov. 30, 2020	Aug. 31, 2020	May 31, 2020	Apr. 30, 2020	Feb. 28, 2019	Dec. 31, 2018	Mar. 31, 2021	Mar. 31, 2020
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Common stock, shares authorized			34,900,878							150,000,000
Dividends declared										$ 0
Common stock, voting rights										Each share of common stock entitles the holder to one vote and to receive dividends when and if declared by the board of directors of the Company.
Shares sold/issued			29,381,125							43,516,125
Losses attributed to noncontrolling interests										$ 1,489,000	$ (190,000)
Aggregate purchase price	[1]										14,250,000
MICA
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued			3,367,804
Shares price per share			$ 0.23
Aggregate purchase price			$ 800,000
Ownership interest percentage			35.00%
MICA
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Equity at acquisition		$ 12,800,000
Common Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Additional shares issued										13,685,000
Series A Senior Preferred Stock | MICA
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Equity at acquisition		7,100,000
Junior Preferred and Common Stockholders | Noncontrolling Interest in Subsidiaries | MICA
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Equity at acquisition		$ 5,700,000
Cullinan Apollo Corp
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Losses attributed to noncontrolling interests										$ 0	0
Cullinan Apollo Corp | Series A Preferred Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued									7,000,000
Gross proceeds									$ 7,000,000
Shares price per share									$ 1
Cullinan Apollo Corp | Series A Preferred Stock | Maximum
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued									11,000,000
Cullinan Pearl Corp | Subscription Agreement between Company and Pearl | Common Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares price per share				$ 0.44
Aggregate purchase price				$ 1,200,000
Purchase of shares				2,730,227
Cullinan Pearl Corp | Taiho Ventures’ Preferred Stock Ownership in Pearl | HLBV
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Losses attributed to noncontrolling interests										1,700,000	(200,000)
Cullinan Pearl Corp | Series A Preferred Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued					9,000,000
Gross proceeds					$ 9,000,000
Cullinan Pearl Corp | Series A Preferred Stock | Taiho Ventures, LLC
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued								14,000,000
Gross proceeds								$ 14,000,000
Shares price per share								$ 1
Cullinan Pearl Corp | Series A Preferred Stock | Taiho License Agreement
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued					1,206,000			1,860,000
Aggregate purchase price								$ 0
Additional consideration received					$ 0
Cullinan Pearl Corp | Series A Preferred Stock | Maximum | Taiho Ventures, LLC
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued								23,000,000
Cullinan Amber Corp
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Losses attributed to noncontrolling interests										0	0
Cullinan Amber Corp | License Agreement between Amber and Massachusetts Institute of Technology
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued							400,132
Cullinan Amber Corp | License Agreement between Amber and Massachusetts Institute of Technology | Common Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Additional consideration received							$ 0
Cullinan Amber Corp | Series A Preferred Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Gross proceeds							$ 3,000,000
Shares issued							3,000,000
Closing price per share							$ 1
Cullinan Amber Corp | Series A Preferred Stock | Maximum
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Sale of stock							9,000,000
Cullinan Florentine Corp
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Losses attributed to noncontrolling interests										0	$ 0
Cullinan Florentine Corp | Tübingen License Agreement
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares sold/issued							725,118
Cullinan Florentine Corp | Tübingen License Agreement | Common Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Additional consideration received							$ 0
Additional shares issued			381,810
Cullinan Florentine Corp | Tübingen License Agreement | Common Stock | DFKZ and UFE Anti-dilution Shares
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Additional consideration received			$ 0
Cullinan Florentine Corp | Series A Preferred Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Gross proceeds			$ 6,000,000				$ 6,000,000
Sale of stock							6,000,000
Closing price per share							$ 1
Additional shares issued			6,000,000
Cullinan Florentine Corp | Series A Preferred Stock | Maximum
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Sale of stock							6,000,000
MICA | HLBV
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Losses attributed to noncontrolling interests										$ 200,000
MICA | Series A Senior Preferred Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares price per share						$ 1.31
Shares issued						6,088,282
Number of share purchased as part of asset acquisition						5,385,787
[1]	Changes in equity activities for the three months ending March 31, 2020 were retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the consolidated financial statements for additional details.

Equity-Based Compensation - Add

Equity-Based Compensation - Additional Information (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Equity-based compensation expense | $	$ 3,508
2021 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved	12,546,386
Split-adjusted restricted stock issued	320,228
Common stock issuable upon exercise of stock options	4,616,187
Common stock issuable upon exercise of stock options, weighted average exercise price | $ / shares	$ 4.30
Increase in number of shares reserved and available for issuance percentage	5.00%
2021 Stock Plan | Executive Officers, Employees and Affiliates
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock issuable upon exercise of stock options	1,753,316
ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved	416,665
Increase in number of shares reserved and available for issuance percentage	1.00%
Increase in number of shares reserved and available for issuance	833,330
ESPP | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Equity-based compensation expense | $	$ 100

Equity-Based Compensation - Sch

Equity-Based Compensation - Schedule of Equity-based Compensation Expense Categories In Consolidated Statements of Operations and Comprehensive Income (Loss) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total equity-based compensation	$ 3,508
Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total equity-based compensation	1,569
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total equity-based compensation	$ 1,939

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Royalty percentage		1.00%
Royalties under obligation		$ 0		$ 0
Globeways Holdings Limited | 2020 Consulting Agreement
Related Party Transaction [Line Items]
Related party transaction, description of transaction		In April 2020, the Company entered into a consulting agreement, or the 2020 Consulting Agreement, with Globeways Holdings Limited, or Globeways. Globeways and entities affiliated with F2 Ventures beneficially own in the aggregate greater than five percent of the Company’s outstanding units and Globeways is beneficially owned by a member of the Company’s board of directors.
Consulting fee per month	$ 25,000
MPM Oncology Charitable Foundation, Inc.
Related Party Transaction [Line Items]
Royalty percentage		0.50%
UBS Optimus Foundation
Related Party Transaction [Line Items]
Royalty percentage		0.50%
Maximum
Related Party Transaction [Line Items]
Management and advisory services		$ 0	$ 100,000
Maximum | Globeways Holdings Limited | 2020 Consulting Agreement
Related Party Transaction [Line Items]
Consulting fee		100,000
Maximum | MPM Capital
Related Party Transaction [Line Items]
General and administrative expense		$ 100,000	$ 100,000

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 0	$ 0
Benefits of federal and state deferred tax assets recognized	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) $ in Millions	1 Months Ended	3 Months Ended
	Dec. 31, 2017ft²	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)
Lessee Lease Description [Line Items]
Rent expense | $		$ 0.2	$ 0.2
Operating lease commence date	Feb. 1, 2018
Operating lease expiration date		Jun. 30, 2024
Massachusetts
Lessee Lease Description [Line Items]
Operating lease rentable area | ft²	7,531

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Payments Due Under Operating Lease (Details) $ in Thousands	Mar. 31, 2021USD ($)
Operating Lease Liabilities Payments Due [Abstract]
Remainder of 2021	$ 451
2022	608
2023	618
2024	313
Future minimum payments due under operating lease	$ 1,990

Employee Benefit Plan - Additio

Employee Benefit Plan - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Maximum | 401(K) Plan
Defined Benefit Plan Disclosure [Line Items]
Expense relating to matching contribution	$ 100,000	$ 100,000

Net Loss per Share - Summary of

Net Loss per Share - Summary of Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (70)	$ (4,973)
Denominator
Total weighted-average shares used in computing net loss per share, basic and diluted	41,977,336	18,747,004
Net Loss per Share:	$ 0	$ (0.27)

Net Loss per Share - Summary _2

Net Loss per Share - Summary of Outstanding Equity Instruments Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	6,853,465	478,293
Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	242,411	478,293
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	6,600,953
Estimated Shares Issuable Under The ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	10,101