Cullinan Oncology (CGEM) 10-K 14 Mar 24 2023 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Mar. 01, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Entity Registrant Name	CULLINAN ONCOLOGY, INC.
Current Fiscal Year End Date	--12-31
Document Annual 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 1350
Entity Address, City or Town	Cambridge
Entity Address, State or Province	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 Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 374.1
Entity Common Stock, Shares Outstanding		43,065,645
Auditor Name	KPMG LLP
Auditor Location	Boston, Massachusetts
Auditor Firm ID	185

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 98,434	$ 156,152
Short-term investments	368,633	311,140
Prepaid expenses and other current assets	13,124	7,180
Total current assets	480,191	474,472
Property and equipment, net	989	1,174
Operating lease right-of-use assets	2,543	4,130
Other assets	459	459
Long-term investments	0	80,882
Total assets	484,182	561,117
Current liabilities:
Accounts payable	2,493	2,660
Accrued expenses and other current liabilities	24,204	14,135
Income tax payable	0	4,282
Operating lease liabilities, current	1,440	1,421
Total current liabilities	28,137	22,498
Long-term liabilities:
Lease liabilities, non-current	2,150	3,590
Total liabilities	30,287	26,088
Commitments and contingencies (Note 12)
Stockholders' equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of December 31, 2023 and 2022; 647,500 and no shares issued and outstanding as of December 31, 2023 and 2022, respectively.	0	0
Common stock, $0.0001 par value, 150,000,000 shares authorized as of December 31, 2023 and 2022; 42,900,083 and 45,796,449 shares issued and outstanding as of December 31, 2023 and 2022, respectively.	4	5
Additional paid-in capital	654,685	585,320
Accumulated other comprehensive loss	(129)	(2,601)
Accumulated deficit	(200,857)	(47,695)
Total Cullinan stockholders' equity	453,703	535,029
Noncontrolling interests	192	0
Total stockholders' equity	453,895	535,029
Total liabilities and stockholders' equity	$ 484,182	$ 561,117

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred Stock, Shares Authorized	10,000,000	10,000,000
Preferred Stock, Shares Issued	647,500	0
Preferred Stock, Shares Outstanding	647,500	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	42,900,083	45,796,449
Common stock, shares outstanding	42,900,083	45,796,449

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Operating expenses:
Research and development	$ 148,156	$ 91,948
General and administrative	42,493	40,189
Total operating expenses	190,649	132,137
Impairment of long-lived assets	440	0
Gain on sale of Cullinan Pearl	0	276,785
Income (loss) from operations	(191,089)	144,648
Other income (expense):
Interest income	21,627	6,611
Other income (expense), net	239	57
Net income (loss) before income taxes	(169,223)	151,316
Income tax expense (benefit)	(14,122)	42,121
Net income (loss)	(155,101)	109,195
Net loss attributable to noncontrolling interests	(1,939)	(2,019)
Net income (loss) attributable to common stockholders of Cullinan	(153,162)	111,214
Comprehensive income (loss):
Net income (loss)	(155,101)	109,195
Unrealized gain (loss) on investments	2,472	(1,763)
Comprehensive income (loss)	(152,629)	107,432
Comprehensive loss attributable to noncontrolling interests	(1,939)	(2,019)
Comprehensive income (loss) attributable to Cullinan	$ (150,690)	$ 109,451
Net income (loss) per share attributable to common stockholders of Cullinan:
Basic	$ (3.69)	$ 2.46
Diluted	$ (3.69)	$ 2.38
Weighted-average shares used in computing net income (loss) per share attributable to common stockholders of Cullinan:
Basic	41,550	45,164
Diluted	41,550	46,640

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Noncontrolling Interests in Subsidiaries
Balances at Dec. 31, 2021	$ 425,374		$ 4	$ 584,714	$ (838)	$ (158,909)	$ 403
Balances, shares at Dec. 31, 2021			44,292,102
Net issuance of common stock under equity-based compensation plans, (In Shares)			1,504,347
Net issuance of common stock under equity-based compensation plans, (Amount)	6,020		$ 1	6,019
Contributions from noncontrolling interests	1,527						1,527
Acquisition of noncontrolling interests	(33,281)			(32,706)			(575)
Equity-based compensation	27,957			27,293			664
Unrealized loss on investments	(1,763)				(1,763)
Net income (loss)	109,195					111,214	(2,019)
Balances at Dec. 31, 2022	535,029		$ 5	585,320	(2,601)	(47,695)	0
Balances, shares at Dec. 31, 2022			45,796,449
Issuance of common stock, Share			3,310,000
Issuance of common stock, Amount	38,388			38,388
Issuance of preferred stock in exchange for common stock, Share		647,500	(6,475,000)
Issuance of preferred stock in exchange for common stock, Amount			$ (1)	1
Net issuance of common stock under equity-based compensation plans, (In Shares)			268,634
Net issuance of common stock under equity-based compensation plans, (Amount)	538			538
Contributions from noncontrolling interests	2,131						2,131
Acquisition of noncontrolling interests	0
Equity-based compensation	30,438			30,438
Unrealized gain/loss on investments	2,472				2,472
Unrealized loss on investments	2,472
Net income (loss)	(155,101)					(153,162)	(1,939)
Balances at Dec. 31, 2023	$ 453,895		$ 4	$ 654,685	$ (129)	$ (200,857)	$ 192
Balances, shares at Dec. 31, 2023		647,500	42,900,083

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Operating activities:
Net income (loss)	$ (155,101)	$ 109,195
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Equity-based compensation expense	30,438	27,957
Amortization (accretion) on marketable securities	(10,154)	1,294
Impairment of long-lived assets	440	0
Depreciation and amortization	310	93
Non cash contributions from noncontrolling interests	131	374
Gain on sale of Cullinan Pearl	0	(276,785)
Realized loss on marketable securities	0	109
Gain on disposal of fixed assets	0	(77)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(5,944)	(1,130)
Accounts payable	(167)	(509)
Accrued expenses and other liabilities	10,054	8,533
Income tax payable	(4,282)	4,282
Net cash used in operating activities	(134,275)	(126,664)
Investing activities:
Proceeds from maturities of marketable securities	409,397	352,933
Purchase of Marketable securities	(373,383)	(377,916)
Purchases of property and equipment	(208)	(1,133)
Sale of Cullinan Pearl, net of cash transferred with sale of $2,898	0	275,000
Proceeds from sale of property and equipment	0	91
Net cash provided by investing activities	35,806	248,975
Financing activities:
Proceeds from issuance of common stock	38,388	0
Proceeds from net issuance of convertible notes	1,825	2,375
Proceeds from net issuance of common stock under equity-based compensation plans	538	6,020
Acquisition of noncontrolling interests	0	(33,281)
Repayment of convertible note	0	(2,200)
Contributions from noncontrolling interests	0	1,153
Net cash provided by (used in) financing activities	40,751	(25,933)
Net increase (decrease) in cash and cash equivalents	(57,718)	96,378
Cash and cash equivalents at beginning of period	156,152	59,774
Cash and cash equivalents at end of period	98,434	156,152
Noncash financing activities
Conversion of convertible note into noncontrolling interest	2,000	0
Purchases of property and equipment included in accounts payable and accrued expenses and other liabilities	0	71
Cash paid (refunded) for income taxes	$ (4,433)	$ 37,801

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Statement of Cash Flows [Abstract]
Cash Transferred with Sale	$ 2,898

Nature of Business and Basis of

Nature of Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2023
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 clinical-stage biopharmaceutical company focused on developing modality-agnostic targeted oncology therapies that was incorporated in September 2016 and has a principal place of business in Cambridge, Massachusetts. Liquidity The Company has incurred significant operating losses, with the exception of 2022, and negative cash flows from operations since its inception and expects to continue to generate operating losses for the foreseeable future. Cullinan’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. Cullinan 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 Cullinan’s expectations for a number of reasons, including reasons beyond the Company’s control. Since inception, Cullinan has funded its operations primarily through the sale of equity securities and from licensing or selling the rights to its product candidates. The Company expects that its cash, cash equivalents, short-term investments, and interest receivable of $ 468.3 million as of December 31, 2023 , will be sufficient to fund its operating expenses and capital expenditure requirements through the next twelve months from the date of issuance of these consolidated financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	(2) Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and in accordance with applicable rules and regulations of the Securities and Exchange Commission ("SEC") for financial reporting and include accounts of the Company and its consolidated subsidiaries. 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"). Under the VOE, the Company consolidates the entity if it determines 1) that it directly, or indirectly, has greater than 50% of the voting shares or other equity holders do not have substantive voting, participation, or liquidation rights, or 2) when the company has a controlling financial interest through its control of the board of directors, and the significant decisions of the entity are made at the board level. The Company concluded that none of its subsidiaries is a VIE and has consolidated each of its subsidiaries under the VOE. Intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of Cullinan’s consolidated financial statements and accompanying notes in conformity with U.S. GAAP requires the Company’s management to make estimates and judgments that affect the amounts reported in the financial statements. On an ongoing basis, Cullinan’s management evaluates its estimates, which include, but are not limited to, estimates related to prepaid and accrued research and development expenses, equity-based compensation, income taxes, and the fair value of royalty transfer agreements. Management's estimates could change period to period based on changes in facts and circumstances. The Company’s management bases its estimates on historical experience and on other relevant assumptions that are believed to be reasonable. Actual results may differ materially from these estimates. Segments Cullinan has determined that its Chief Executive Officer is the Chief Operating Decision Maker ("CODM"). The Company operates and manages the business as one reporting and one operating segment, which is the business of developing early-stage cancer therapeutics. Cullinan’s CODM reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets were located in the U.S. as of each of December 31, 2023 and 2022. Expenditures for additions to long-lived assets included purchases of property and equipment in each of 2023 and 2022 . Concentration of Risk Cullinan had no significant concentration of credit risk as of December 31, 2023. Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. as of December 31, 2023. Deposits at banks may exceed the insurance provided on such deposits. These deposits may be redeemed upon demand, and therefore, bear minimal risk. Under our investment policy, the Company limits amounts invested in such securities by investment type, credit rating, maturity, industry group and issuer. The goals of our investment policy are (i) safety and preservation of principal and diversification of risk and (ii) liquidity of investments sufficient to meet cash flow requirements. Cullinan is subject to certain risks and uncertainties and believes that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: ability to conduct and complete preclinical and clinical trials of our current and future product candidates; ability to obtain future financing; ability to build a successful pipeline of product candidates, including efficient expenditures of its resources; regulatory approval and market acceptance of, and reimbursement for, current and future product candidates; protection of Cullinan’s intellectual property, including litigation or claims against the Company based on intellectual property, patent, product, regulatory or other factors; performance of third-party clinical research organizations and manufacturers upon which the Company relies; and Cullinan’s ability to attract and retain employees necessary to support its growth. The Company is dependent and expects to continue to be dependent on a small number of third-party manufacturers to supply drug product and drug substance for research and development activities in its programs. These programs could be adversely affected by a significant interruption in supply. 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 each of December 31, 2023 and 2022 , cash equivalents consist of government-backed money market funds. Investments Cullinan generally holds investments in marketable securities. Investments not classified as cash equivalents with maturities of less than twelve months are classified as short-term investments in the consolidated balance sheets. Investments 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. The amortized cost of marketable securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion is included in interest income. Dividends are also included in interest income. Interest receivable is included in prepaid expenses and other current assets on the consolidated balance sheets and represents accrued and unpaid interest on Cullinan's marketable securities. The Company periodically reviews its marketable securities for impairment and adjusts these investments to their fair value when a decline in market value is deemed to be other than temporary. Declines in fair value judged to be other-than-temporary on marketable securities, if any, are included in other income (expense), net. Fair Value of Financial Instruments Cullinan has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements. The three levels of the fair value hierarchy are described below: Level 1—Unadjusted quoted prices for identical assets or liabilities in active markets that the Company has the ability to access; Level 2—Quoted prices for similar assets and liabilities in active markets or other market-observable inputs such as interest rates, yield curves and foreign currency spot rates; and Level 3—Pricing or valuations that require inputs that are both significant to the fair value measurement and unobservable. There were no transfers of financial assets or liabilities measured at fair value between Level 1 and Level 2, and there were no Level 3 investments during 2023 or 2022. Cullinan's financial assets recorded at fair value consist of investments. The fair value of the Company’s investments is primarily determined using market quotations or prices obtained from independent pricing sources. As of December 31, 2023 and 2022 , the fair values of cash and cash equivalents, prepaid expenses and other current assets, accounts payable, accrued expenses and other current liabilities approximated their carrying values due to the short-term nature of these instruments . Property and Equipment, net Property and equipment is stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Asset Class Estimated Useful Life Office furniture and equipment 5 years Leasehold improvements Shorter of the useful life of the asset or the lease term Maintenance and repairs that do not improve or extend the life of the assets are expensed when incurred. Upon disposal or retirement of assets, the cost and accumulated depreciation and amortization are removed from the consolidated balance sheets and any resulting gain or loss is reflected in the consolidated statements of operations and comprehensive income (loss). Leases Cullinan determines if an arrangement is a lease at inception. A contract is or contains a lease if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company classifies leases at the lease commencement date as operating or finance leases and records a right-of-use asset ("ROU") and a lease liability on the consolidated balance sheets for all leases with an initial lease term of greater than 12 months. Leases with an initial term of 12 months or less are not recorded in the balance sheet, and payments are recognized as expense on a straight-line basis over the lease term. Cullinan enters into contracts that contain both lease and non-lease components. Non-lease components may include maintenance, utilities and other operating costs. The Company combines the lease and non-lease components of fixed costs in its lease arrangements as a single lease component. Variable costs, such as utilities or maintenance costs, are not included in the measurement of ROU assets and lease liabilities but rather are expensed when the event determining the amount of variable consideration to be paid occurs. Operating lease assets and liabilities are recognized at the lease commencement date based on the present value of the lease payments over the lease term using the discount rate implicit in the lease. If the discount rate is not readily determinable, Cullinan utilizes an estimate of its incremental borrowing rate based upon the available information at the lease commencement date. Operating lease assets are further adjusted for prepaid or accrued lease payments. Operating lease payments are expensed using the straight-line method as an operating expense over the lease term. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that Cullinan will exercise that option. Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to future undiscounted net cash flows expected to be generated. Impairment charges are recognized at the amount by which the carrying amount of an asset exceeds the fair value of the asset. Assets to be disposed of are reported at the lower of the carrying amount or the fair value less costs to sell. Noncontrolling Interests Noncontrolling interests represent third-party interests in the Company’s subsidiaries. Cullinan determines the amount of the noncontrolling interests in the net assets of the Company’s subsidiaries at each balance sheet date using the hypothetical liquidation at book value ("HLBV") method. Under the HLBV method, the amounts reported as noncontrolling interests in the consolidated balance sheets represent the amounts third parties would hypothetically receive at each balance sheet date under the liquidation provisions of the subsidiaries, assuming the net assets of the subsidiaries were liquidated at their recorded amounts determined in accordance with U.S. GAAP and distributed to the owners of the subsidiaries. Net income (loss) attributable to noncontrolling interests on the consolidated statements of operations and comprehensive income (loss) is determined as the difference in the noncontrolling interest in the consolidated balance sheets between the start and end of each reporting period, after taking into account any capital transactions between the subsidiaries and third parties. Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services. The amount of revenue recognized reflects the consideration to which Cullinan expects to be entitled in exchange for these goods and services. To achieve this core principle, the Company applies the following five steps: 1) identify the customer contract; 2) identify the contract’s performance obligations; 3) determine the transaction price; 4) allocate the transaction price to the performance obligations; and 5) recognize revenue when or as a performance obligation is satisfied. Licensing arrangements are analyzed to determine whether the promised goods or services, which could include licenses and research and development materials and services, are distinct or whether they must be accounted for as part of a combined performance obligation. The transaction price is determined based on the consideration to which Cullinan will be entitled. The transaction price may include fixed amounts, variable amounts, or both. The Company reevaluates the probability of realizing such variable consideration and any related constraints at each reporting period. Cullinan includes variable consideration in the transaction price to the extent it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur. The Company allocates the transaction price based on the estimated standalone selling price of the underlying performance obligations. Cullinan must develop assumptions that require judgment to determine the standalone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the standalone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs to complete the respective performance obligation. Cullinan also utilizes judgment in assessing whether or not variable consideration is constrained or if it can be allocated specifically to one or more performance obligations in the arrangement. When a performance obligation is satisfied, revenue is recognized for the amount of the transaction price allocated to that performance obligation on a relative standalone selling price basis, which excludes estimates of variable consideration that are constrained. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. For performance obligations consisting of licenses and other promises, Cullinan utilizes judgment to assess whether the combined performance obligation is satisfied over time or at a point in time and the recognition pattern for the portion of the transaction price allocated to the performance obligation. Research and Development Expenses Research and development costs are expensed as incurred. Research and development expenses consist primarily of employee compensation costs and amounts incurred with 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 Cullinan 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. Cullinan has entered into various research and development related contracts with parties both inside and outside of the U.S. The payments related to these agreements are recorded as research and development expenses as incurred. The Company records accrued liabilities for estimated ongoing research and development costs. When evaluating the adequacy of the accrued liabilities, Cullinan 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. 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 using the fair value of the awards and is recognized as expense over the requisite service period, which is generally the vesting period. Forfeitures are recognized as they occur. Cullinan 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. The fair value of service-based restricted stock units ("RSUs") is the closing market price of the Company's common stock on the grant date. The fair value of market-based RSUs is measured on the grant date using a Monte Carlo simulation model. Cullinan estimated the fair value of stock options using the Black-Scholes option pricing model. Both the Monte Carlo simulation model and the Black-Scholes option pricing model require the input of objective and subjective assumptions. Certain assumptions used, including the Company’s expected stock price volatility, 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. Prior to 2023, the expected volatility used in the Black-Scholes option pricing model for new options was based on historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption. In 2023, Cullinan determined that a sufficient amount of historical information was available regarding the volatility of its stock price to begin using a blended rate that combines the Company's historical volatility with the historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption. Income Taxes Cullinan recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount of benefit that is greater than fifty percent likely to be realized upon settlement. Changes in measurement are reflect in the period in which the change in judgment occurs. Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from nonowner sources. The Company’s only element of other comprehensive loss is unrealized gains and losses on investments. Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan Basic net income (loss) per share attributable to common stockholders of Cullinan is determined by dividing net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period. Diluted net income (loss) per share attributable to common stockholders of Cullinan is determined by dividing earnings (net loss) attributable to common stockholders by the weighted-average number of common shares outstanding during the period, adjusted for the dilutive effect of shares of common stock equivalents as determined using the treasury stock method for equity awards and the if-converted method for preferred stock. Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. Cullinan has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these consolidated financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. The JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies. The Company expects to use the extended transition period for any other new or revised accounting standards during the period in which it remains an emerging growth company. Recently Issued Accounting Pronouncements In November 2023, the Financial Accounting Standards Board (the “FASB”) issued an accounting standards update to improve reportable segment disclosure requirements, primarily through enhanced disclosures about significant segment expenses. The main provisions of this update require companies to disclose, on an annual and interim basis, significant segment expenses, segment profit and loss, and other segments items that are regularly provided to the CODM. This update also requires companies to disclose the title and position of the CODM and to explain how the CODM uses the reported segment measures in assessing segment performance and deciding how to allocate resources. The update also requires companies with a single reportable segment to provide all required segment reporting disclosures. This new standard will be effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted. Cullinan adopted this standard on January 1, 2024 for 2024 annual reporting and interim periods beginning in 2025. This new standard will not have a material impact on the Company’s consolidated financial statements and associated disclosures. In December 2023, the FASB issued an accounting standards update to enhance transparency about income tax information through improvements to income tax disclosures primarily related to the rate reconciliation and income taxes paid information. The main provisions in this update will require companies to disclose, on an annual basis, specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. This update will also require companies to disclose, on an annual basis, the amount of income taxes paid, income (or loss) from continuing operations before income tax expense (or benefit), and income tax expense (or benefit) from continuing operations, disaggregated between federal, state, foreign, and jurisdictional taxes. This new standard will be effective beginning for fiscal years beginning after December 15, 2025, and early adoption is permitted. The Company expects that it will adopt this new standard on January 1, 2026. The Company is evaluating the impact this new standard will have on its consolidated financial statements and associated disclosures.

Sale of Cullinan Pearl

Sale of Cullinan Pearl	12 Months Ended
	Dec. 31, 2023
Debt Securities, Available-for-Sale [Abstract]
Sale of Cullinan Pearl	(3) Sale of Cullinan Pearl In June 2022, Cullinan sold its equity interest in its development subsidiary, Cullinan Pearl Corp. (“Cullinan Pearl”), which had worldwide rights to zipalertinib (CLN-081/TAS6417), excluding Japan, mainland China, Hong Kong, Macau and Taiwan, to Taiho for an upfront payment of $ 275.0 million, with an increase to the purchase price in the amount of $ 2.9 million for cash held by Cullinan Pearl that was transferred with the sale. Pursuant to the share purchase agreement with Taiho, Cullinan is also eligible to receive up to an additional $ 130.0 million tied to epidermal growth factor receptor exon 20 non-small-cell lung cancer regulatory milestones. Cullinan concluded the transaction was a sale of non-financial assets, which comprised mainly of intellectual property rights and related intangible assets, and that it transferred control of the non-financial assets at the closing of the sale. Cullinan recognized a gain on sale of Cullinan Pearl of $ 276.8 million within income from operations in its consolidated statements of operations and other comprehensive income (loss) for 2022. The table below sets forth the book value of the Cullinan Pearl assets and liabilities sold along with the calculation of the gain on sale based on the cash consideration received (in thousands): Book value of assets sold Cash $ 2,898 Prepaid expenses and other current assets 619 Amounts attributable to assets sold 3,517 Book value of liabilities sold Accrued expenses and other current liabilities 2,404 Amounts attributable to liabilities sold 2,404 Total identifiable net assets sold 1,113 Upfront consideration, inclusive of cash transferred of $ 2,898 277,898 Gain on sale of Cullinan Pearl $ 276,785 During 2022 , Cullinan Pearl issued $ 2.2 million of convertible notes to an affiliate of Taiho. Cullinan repaid these convertible notes at the closing of the Cullinan Pearl sale.

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2023
Financial Instruments Pledged as Collateral [Abstract]
Financial Instruments	(4) Financial Instruments Investments Cullinan recognized its investments by security type at December 31, 2023 as follows (in thousands): Amortized Gross Gross Estimated Short-term investments U.S. government notes $ 208,289 $ 221 $ ( 16 ) $ 208,494 Corporate notes 99,359 27 ( 275 ) 99,111 Asset-backed securities 61,114 3 ( 89 ) 61,028 Total short-term investments 368,762 251 ( 380 ) 368,633 Total investments $ 368,762 $ 251 $ ( 380 ) $ 368,633 Cullinan recognized its investments by security type at December 31, 2022 as follows (in thousands): Amortized Gross Gross Estimated Short-term investments Corporate notes $ 244,498 $ 11 $ ( 1,743 ) $ 242,766 U.S. government notes 34,029 — ( 290 ) 33,739 Commercial paper 18,035 3 ( 13 ) 18,025 Asset-backed securities 16,625 — ( 15 ) 16,610 Total short-term investments 313,187 14 ( 2,061 ) 311,140 Long-term investments Corporate notes 81,436 18 ( 572 ) 80,882 Total long-term investments 81,436 18 ( 572 ) 80,882 Total investments $ 394,623 $ 32 $ ( 2,633 ) $ 392,022 Fair Value of Financial Instruments The following table sets forth the fair value of Cullinan’s financial assets that were measured at fair value on a recurring basis as of December 31, 2023 (in thousands): Level 1 Level 2 Level 3 Total Short-term investments U.S. government notes $ — $ 208,494 $ — $ 208,494 Corporate notes — 99,111 — 99,111 Asset-backed securities — 61,028 — 61,028 Total short-term investments — 368,633 — 368,633 Total investments $ — $ 368,633 $ — $ 368,633 The following table sets forth the fair value of Cullinan’s financial assets that were measured at fair value on a recurring basis as of December 31, 2022 (in thousands): Level 1 Level 2 Level 3 Total Short-term investments Corporate notes $ — $ 242,766 $ — $ 242,766 U.S. government notes — 33,739 — 33,739 Commercial paper — 18,025 — 18,025 Asset-backed securities — 16,610 — 16,610 Total short-term investments — 311,140 — 311,140 Long-term investments Corporate notes — 80,882 — 80,882 Total long-term investments — 80,882 — 80,882 Total investments $ — $ 392,022 $ — $ 392,022

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and Equipment, net	(5) Property and Equipment, net Property and equipment, net consisted of the following as of December 31, 2023 and 2022 (in thousands): December 31, 2023 2022 Office furniture and equipment $ 765 $ 681 Leasehold improvements 576 628 Total property and equipment, gross 1,341 1,309 Less: accumulated depreciation ( 352 ) ( 135 ) Total property and equipment, net $ 989 $ 1,174 Depreciation expense was $ 0.3 million and $ 0.1 million for 2023 and 2022 , respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2023
Accrued Expenses And Other Current Liabilities [Abstract]
Accrued Expenses and Other Current Liabilities	(6) Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following as of December 31, 2023 and 2022 (in thousands): December 31, 2023 2022 Contracted research and development expenses $ 8,434 $ 7,486 Due to Taiho under collaboration agreement, net 7,869 — Employee compensation 6,987 4,516 Other current liabilities 914 1,955 Convertible note and accrued interest — 178 $ 24,204 $ 14,135

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2023
License And Collaboration Agreements [Abstract]
License and Collaboration Agreements	(7) License and Collaboration Agreements Harbour License Agreement In February 2023, the Company and Harbour BioMed US Inc. (“Harbour”) entered into a license and collaboration agreement (the “Harbour License Agreement”), pursuant to which Harbour granted to Cullinan an exclusive license for the development, manufacturing and commercialization of HBM7008 (CLN-418) in the U.S. Under the terms of the Harbour License Agreement, Cullinan paid Harbour an upfront license fee of $ 25.0 million at signing. Harbour is eligible to receive up to $ 148.0 million in milestone payments based on the achievement of pre-specified development and regulatory milestones. Harbour is also eligible to receive up to an additional $ 415.0 million in sales-based milestones as well as tiered royalties up to the high teens on a licensed product-by-licensed product basis, as a percentage of U.S. commercial sales. In addition, under the Harbour License Agreement, Harbour granted Cullinan certain intellectual property rights to enable the Company to perform its obligations and exercise its rights under the Harbour License Agreement. As of December 31, 2023, no milestones have been achieved under the Harbour License Agreement. Unless earlier terminated, the Harbour License Agreement will continue in effect until the expiration of Cullinan’s royalty obligations. The Harbour License Agreement may be terminated by either party for a material breach by the other party, subject to notice and cure provisions, or in the event of the other party’s insolvency. Cullinan may terminate the Harbour License Agreement for convenience by providing 90 days written notice to Harbour. In the Harbour License Agreement, each party made customary representations and warranties and agreed to customary covenants, including, without limitation, with respect to indemnification, for transactions of this type. Cullinan evaluated the Harbour License Agreement and determined that the exclusive license for the development, manufacturing and commercialization of HBM7008 (CLN-418) in the U.S represented an asset acquisition of in-process research and development. The Company also determined that the asset had no alternative future use at the time of acquisition, and therefore the upfront license fee of $ 25.0 million was recorded within research and development expenses for 2023. Co-Development Agreement with Taiho In June 2022, concurrently with the closing of the sale of the Company’s equity interest in Cullinan Pearl, the Company entered into a co-development agreement with an affiliate of Taiho, pursuant to which the Company is collaborating to develop zipalertinib and has retained the option to co-commercialize zipalertinib in the U.S. Development costs for zipalertinib incurred after the sale of the Company’s equity interest in Cullinan Pearl are shared equally between Taiho and the Company with each party receiving 50 % of any future pre-tax profits from potential U.S. sales of zipalertinib. The Company concluded that the co-development agreement with Taiho is a collaborative arrangement because Cullinan is an active participant in the development of zipalertinib. Payments made to or received from Taiho for zipalertinib development activities after the execution of the co-development agreement are recorded within research and development expenses. For 2023 and 2022 , the Company recorded research and development expense of $ 25.3 million and $ 2.5 million, respectively, related to its share of costs incurred by Taiho. Cullinan incurred $ 6.9 million and $ 3.5 million of costs that were reimbursable by Taiho during 2023 and 2022 , respectively, which were recorded as a reduction to research and development expenses. The net amount of $ 7.9 million due to Taiho was recorded within accrued expenses and other current liabilities as of December 31, 2023. The net amount of $ 1.0 million due from Taiho was recorded within prepaid expenses and other current assets as of December 31, 2022. DKFZ/Tübingen License Agreement The Company has exclusive worldwide rights to CLN-049, its bispecific antibody targeting FLT3 and CD3, pursuant to an exclusive license agreement (the "DKFZ/Tübingen License Agreement") with Deutsches Krebsforschungszentrum ("DKFZ"), Eberhard Karls University of Tübingen, Faculty of Medicine, and Universitätsmedizin Gesellschaft für Forschung und Entwicklung mbH, Tübingen. Pursuant to the DKFZ/Tübingen License Agreement, DKFZ and University of Tübingen, collectively referred to as the Licensor, granted to Cullinan an exclusive worldwide, milestone- and royalty-bearing license under certain licensed patent rights, applications, technical information and know-how, with the right to grant sublicenses through multiple tiers to research, develop, commercialize or otherwise exploit licensed products within the field. The Company shall pay certain non-refundable, non-creditable milestone payments to the Licensor upon the occurrence of certain clinical and regulatory events related to a licensed product. Each milestone payment is paid one time only up to a certain payment amount. Furthermore, Cullinan is required to pay running low to mid-single digit royalty percentage on net sales of each licensed product on a country-by-country and product-by-product basis during the royalty term, subject to certain offsets or reductions. The aggregate, worldwide royalties due to Licensor for net sales of any licensed product in a calendar year shall not be reduced to an amount less than low to mid-single digit percentages. Such royalty obligations will expire on a country-by-country and product-by-product basis upon the later of (a) the expiration of the last valid claim of a patent which covers a product in such country and (b) a low double digit anniversary following the first commercial sale of a product in such country. Under certain conditions upon a first change in control in the Company's CLN-049 development subsidiary, Cullinan shall pay a non-refundable, non-creditable mid-single digit percent of sale proceeds, provided, however, that such payment shall not be required following consummation of an initial public offering of the Company's CLN-049 development subsidiary. Either party may terminate the agreement upon a material breach by the other party or insolvency of the other party. Cullinan may terminate the DKFZ/Tübingen License Agreement for any or no reason after the first filing of an investigational new drug application or clinical trial agreement by providing prior written notice. Licensor may terminate the agreement by providing prior written notice, if the Company or any of its affiliates challenges the validity of certain patent rights. Unless earlier terminated, the DKFZ/Tübingen License Agreement continues perpetually. As of December 31, 2023, no milestones have been achieved under the DKFZ/Tübingen License Agreement. Cullinan did not incur license fee expense relating to this agreement during 2023 and 2022. Steinle Agreement The Company has exclusive worldwide rights to CLN-619, its MICA/B-targeted humanized IgG1 monoclonal antibody and has an agreement with Dr. Alexander Steinle (the "Steinle Agreement"), who provided Cullinan with services for the discovery, design and development of monoclonal antibodies that prevent the proteolytic cleavage of MICA from the surface of a cancer cell and augments killing of these cancer cells by immune cells expressing NKG2D receptors (“MICA antibodies”). Under this agreement, the Company shall pay certain non-refundable, non-creditable milestone payments to Dr. Steinle upon the occurrence of certain clinical and regulatory events related to a MICA antibody. Each milestone payment is paid one time only up to a certain payment amount. Cullinan is also required to pay Dr. Steinle a low single digit royalty percentage on net sales of each MICA antibody on a country-by-country and product-by-product basis during the royalty term, subject to certain offsets or reductions. During 2023, Cullinan did not incur any milestone payments under the Steinle Agreement. During 2022 , Cullinan recorded $ 0.1 million related to milestone payments under the Steinle Agreement within research and development expenses. Massachusetts Institute of Technology The Company has exclusive worldwide rights to CLN-617, its fusion protein combining two potent antitumor cytokines, IL-2 and IL-12, with tumor retention domains for the treatment of solid tumors, pursuant to a license agreement with the Massachusetts Institute of Technology ("MIT") to develop a cancer immunotherapy product worldwide (the "MIT License Agreement"). Cullinan is also responsible for paying non-refundable, creditable annual license maintenance fees in an increasing amount over a certain number of years and a fixed amount subsequent to this period of time. In addition, MIT granted to the Company an exclusive option to amend the initially determined field to include expansion fields, and such amendment would trigger the payment to MIT of an amendment fee. Additionally, Cullinan is obligated to pay certain non-refundable, non-creditable milestone payments to MIT upon the achievement by itself or its sublicensees of certain clinical and regulatory milestones in an aggregate amount up to $ 7.0 million for each distinct licensed product. Each milestone payment is paid one time only up to a certain payment amount, except there are separate milestone payments payable for a second and third indication of a licensed product in an aggregate amount up to $ 5.5 million per product. Cullinan shall also pay to MIT certain one-time milestone payments for the achievement of certain commercial milestones based on the calculation of net sales across all licensed products in all indications in an aggregate amount up to $ 12.5 million. As of December 31, 2023 , the Company has incurred a cumulative $ 0.7 million of milestone payments under the MIT License Agreement. Under certain conditions upon a change in control of the Company's CLN-617 development subsidiary, Cullinan is required to pay a specified change in control fee and its CLN-617 clinical and regulatory milestone payments shall be increased by 100 %. Furthermore, the Company is required to pay running low single-digit royalty percentage on net sales of all licensed products for each reporting period, subject to certain offsets or reductions. The royalties due to MIT for net sales of the licensed product shall not be reduced by more than a mid double-digit percentage. Cullinan is also required to share any income from sublicensing the licensed products, with the percentage to be determined by the clinical phase of the licensed product, no greater than low to mid double-digit percentages. Such royalty obligations will expire on a country-by-country and product-by-product basis upon the expiration or abandonment of all issued patents and filed patent applications within the patent rights. During 2023 and 2022 , Cullinan recorded $ 0.8 million and $ 0.4 million, respectively, relating to the MIT License Agreement within research and development expenses. Adimab Cullinan has a collaboration agreement with Adimab, LLC ("Adimab") (the "Adimab Collaboration Agreement"). Pursuant to the Adimab Collaboration Agreement, the Company selected a number of biological targets against which Adimab used its proprietary platform technology to discover and/or optimize antibodies based upon mutually agreed-upon research plans. Under the Adimab Collaboration Agreement, Cullinan has the ability to select a specified number of additional biological targets against which Adimab will provide additional antibody discovery and optimization services. During the research term and evaluation term for a given research program with Adimab, the Company has a non-exclusive worldwide license under Adimab’s technology to perform certain research activities and to evaluate the program antibodies to determine whether Cullinan wants to exercise its option to obtain a royalty-free, fully paid, non-exclusive license to exploit such antibodies and sublicense through multiple tiers (the "Adimab Option"). Under the Adimab Collaboration Agreement, Cullinan paid a one-time, non-creditable, non-refundable technology access fee. Cullinan is also required to pay an annual access fee and research funding fees in connection with Adimab’s full-time employees’ compensation for performance of Adimab’s obligations under the Adimab Collaboration Agreement. Cullinan is also obligated to make certain research delivery, clinical and sales milestone payments to Adimab in an aggregate amount of up to $ 15.8 million for each product, on a product-by-product basis, subject to certain reductions and discounts. As of December 31, 2023 , Cullinan has incurred a cumulative $ 0.5 million of milestone payments under the Adimab Collaboration Agreement. Cullinan is obligated to pay certain royalty payments on a product-by-product basis at a low single-digit percentage of annual aggregate worldwide net sales. Such royalty obligations will expire on a country-by-country and product-by-product basis upon the later of (a) a certain low double-digit number of years after the first commercial sale of such product in such country and (b) the expiration of the last issued and not expired, permanently revoked, or invalid claim within a program patent covering such product. Cullinan may terminate the Adimab Collaboration Agreement at any time, for any reason, upon a specified period advance written notice. The term of the Adimab Collaboration Agreement expires upon the last research program’s evaluation term in the event no Adimab Option is exercised or in the event an Adimab Option is exercised, after the royalty term expires at the later of a specified period or invalid patent coverage of the relevant product. During each of 2023 and 2022 , Cullinan recorded $ 0.5 million relating to the Adimab Collaboration Agreement within research and development expenses.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2023
Stockholders' Equity Note [Abstract]
Stockholders' Equity	(8) Stockholders' Equity Common Stock 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 December 31, 2023. At-the-Market Equity Offering Program In May 2023, Cullinan entered into an agreement with Cowen and Company, LLC (“Cowen”) to establish an at-the-market equity offering program (the “ATM") pursuant to which Cullinan may offer and sell up to $ 125.0 million of its common stock from time to time through Cowen, acting as its sales agent. In 2023 , the Company sold approximately 3.3 million shares under the ATM and received net proceeds of $ 38.4 million after deducting commissions. As of December 31, 2023 , Cullinan had $ 85.6 million in shares of its common stock remaining under the ATM. Preferred Stock In January 2023, the Company entered into an exchange agreement with Biotechnology Value Fund, L.P., Biotechnology Value Fund II, L.P., Biotechnology Value Trading Fund OS LP and MSI BVF SPV, LLC (the “Stockholders”), pursuant to which the Stockholders exchanged 6.5 million shares of Cullinan’s common stock for 0.6 million shares of newly designated Series A convertible preferred stock, a “toothless” preferred stock, par value $ 0.0001 per share. Each share of the preferred stock will be convertible into ten shares of common stock at the option of the holder at any time, subject to certain limitations, including that the holder will be prohibited from converting preferred stock into common stock if, as a result of such conversion, the holder, together with its affiliates, would beneficially own a number of shares of common stock more than 9.99 % of the total common stock then issued and outstanding immediately following the conversion of such shares of preferred stock. Holders of the preferred stock are permitted to increase this percentage to an amount not to exceed 19.99 % upon 60 days notice. Shares of preferred stock will generally have no voting rights, except as required by law and except that the consent of a majority of the holders of the outstanding preferred stock will be required to amend the terms of the preferred stock. In the event of the Company’s liquidation, dissolution or winding up, holders of preferred stock will participate pari passu with any distribution of proceeds to holders of common stock. Holders of preferred stock are entitled to receive when, as, and if dividends are declared and paid on the common stock, an equivalent dividend, calculated on an as-converted basis. Shares of preferred stock are otherwise not entitled to dividends. The preferred stock ranks (i) senior to any class or series of capital stock of Cullinan hereafter created specifically ranking by its terms junior to the preferred stock; (ii) on parity with the common stock and any class or series of capital stock of the Company created specifically ranking by its terms on parity with the preferred stock; and (iii) junior to any class or series of capital stock of Cullinan created specifically ranking by its terms senior to any preferred stock, in each case, as to distributions of assets upon liquidation, dissolution or winding up of the Company, whether voluntarily or involuntarily. The Company evaluated the preferred stock for liability or equity classification. Cullinan determined that the preferred stock should be classified as permanent equity as it is not redeemable for cash or other assets (i) on a fixed or determinable date, (ii) at the option of the holder, or (iii) upon the occurrence of an event that is not solely within control of the Company. Noncontrolling Interests in Subsidiaries Certain of the Company's development subsidiaries have issued common stock and preferred stock to the Company and to third parties. The holders of subsidiary common stock and preferred stock are generally 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 preferred stock of the respective subsidiary. In March 2022, certain existing outside investors purchased equity from the Company's CLN-619 development subsidiary for $ 1.2 million. In October and November 2022, Cullinan purchased equity in the Company's CLN-619 development subsidiary from several of the CLN-619 development subsidiary’s other stockholders for $ 33.3 million. The following table shows the Company’s ownership interest as of December 31, 2023 in its development subsidiaries and their product candidates: Development Subsidiary (Product Candidate) Ownership as of Cullinan MICA Corp. (CLN-619) 95 % Cullinan Florentine Corp. (CLN-049) 96 % Cullinan Amber Corp. (CLN-617) 94 %

Equity-Based Compensation

Equity-Based Compensation	12 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Equity-Based Compensation	(9) Equity-Based Compensation The Company recorded equity-based compensation in the following expense categories in the consolidated statements of operations and comprehensive income (loss) in 2023 and 2022 (in thousands): 2023 2022 General and administrative $ 18,270 $ 16,939 Research and development 12,168 11,018 Total equity-based compensation $ 30,438 $ 27,957 2021 Stock Option and Incentive Plan Cullinan grants equity awards in the form of stock options, restricted stock awards ("RSAs") and RSUs to its employees, directors, consultants and other key persons through the 2021 Stock Option and Incentive Plan (the "2021 Stock Plan"). Cullinan has also granted equity awards outside of the 2021 Stock Plan in the form of stock options as an inducement material to an individual's entering into employment with the Company. As of December 31, 2023 , there were approximately 2.6 million shares remaining for future grants under the 2021 Stock Plan. 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 by 5 % of the outstanding number of shares of Cullinan’s common stock on the immediately preceding December 31 or such lesser number of shares as determined by Cullinan’s board of directors or compensation committee. On January 1, 2024, the total number of shares available for issuance under the 2021 Stock Plan increased by approximately 2.1 million shares under this provision. The options granted have a ten-year term and were issued with an exercise price equal to the closing market price of Cullinan’s common stock on the grant date. For equity awards with service-based vesting conditions, Cullinan recognizes compensation expense over the vesting period, which is generally over a four-year period. For equity awards with a market-based vesting condition, the Company recognizes compensation expense over the requisite service period. The number of shares awarded, if any, when a market-based award vests will depend on the degree of achievement of the corporate stock price metrics within the performance period of the award. Determining fair value of options The fair value of options is estimated using the Black-Scholes option pricing model, which takes into account inputs such as the exercise price, the value of the underlying common stock at the grant date, expected term, expected volatility, risk-free interest rate and dividend yield. The fair value of each grant of options during 2023 and 2022 were determined using the methods and assumptions discussed below: • The expected term of options is determined using the “simplified” method, as prescribed in the SEC Staff Accounting Bulletin (SAB) No. 107, whereby the expected life equals the arithmetic average of the vesting term and the original contractual term of the option due to Cullinan’s lack of sufficient historical data. • The risk-free interest rate is based on implied yields available from U.S. Treasury securities with a remaining term equal to the expected term assumed at the grant date. • Prior to 2023, the expected volatility used in the Black-Scholes option pricing model for new options was based on historical volatilities of the stock prices of similar entities within Cullinan’s industry over a period of time commensurate with the expected term assumption. In 2023, the Company determined that a sufficient amount of historical information was available regarding the volatility of its stock price to begin using a blended rate that combines its historical volatility with the historical volatilities of the stock prices of similar entities within Cullinan’s industry over a period of time commensurate with the expected term assumption. • The estimated annual dividend yield was based on the Company’s expectation of not paying dividends on its common stock in the foreseeable future. For 2023 and 2022 , the weighted-average grant date fair value of the options granted were $ 7.70 and $ 9.02 per share, respectively. The grant date fair value was estimated at the time of grant using the Black-Scholes option-pricing model using the following weighted-average assumptions in 2023 and 2022: 2023 2022 Risk-free interest rate 4.0 % 2.7 % Expected term (in years) 6.0 6.0 Expected volatility 78.7 % 79.8 % Expected dividend yield 0.0 % 0.0 % Determining fair value of market-based RSUs The Company measures the fair value of market-based RSUs on the date of grant using a Monte Carlo simulation model. The Monte Carlo simulation requires the input of assumptions, including Cullinan's stock price, the volatility of its stock price, remaining term in years, expected dividend yield and risk-free rate. The Company used its own trading history to calculate the expected volatility of the market-based RSUs granted. The risk-free interest rate is determined by reference to implied yields available from U.S. Treasury securities with a remaining term equal to the expected term assumed at the grant date. There were no market-based RSUs granted during 2023 . The following table details the assumptions used in the Monte Carlo simulation model used to estimate the fair value of the market-based RSUs granted during 2022: 2022 Stock price $ 12.98 Volatility 82.5 % Expected term (in years) 2.7 Risk-free rate 2.9 % Expected dividend yield 0.0 % Stock options The following table summarizes 2023 and 2022 stock option activity (options and aggregate intrinsic value in thousands): Number of Options Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value Outstanding as of December 31, 2021 9,259 $ 15.77 Granted 2,888 $ 12.96 Exercised ( 1,523 ) $ 4.31 Forfeited ( 1,290 ) $ 13.80 Outstanding as of December 31, 2022 9,334 $ 17.04 Granted 2,091 $ 10.96 Exercised ( 58 ) $ 4.30 Forfeited ( 828 ) $ 22.63 Outstanding as of December 31, 2023 10,539 $ 15.46 7.94 $ 10,753 Exercisable as of December 31, 2023 5,535 $ 15.37 7.49 $ 10,170 As of December 31, 2023 and 2022 , there was $ 50.9 million and $ 66.0 million in unrecognized compensation costs that are expected to be recognized over a remaining weighted-average period of 2.4 and 2.9 years, respectively. The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of Cullinan’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. The total intrinsic value of options exercised in 2023 and 2022 was $ 0.3 million and $ 13.1 million, respectively. RSUs The following table summarizes the activity related to RSUs during 2023 and 2022 (shares in thousands): Number of Shares Weighted-Average Grant Date Fair Value Outstanding unvested as of December 31, 2021 — $ — Granted (1) 502 $ 13.28 Vested ( 46 ) $ 13.60 Forfeited ( 87 ) $ 13.60 Outstanding unvested as of December 31, 2022 369 $ 13.16 Granted 638 $ 11.29 Vested ( 178 ) $ 11.90 Forfeited ( 25 ) $ 12.26 Outstanding unvested as of December 31, 2023 804 $ 11.98 (1) The number granted represents the number of shares issuable upon vesting of service-based and market-based RSUs, assuming the Company achieves its corporate stock price metrics at the target achievement level. As of December 31, 2023 and 2022 , there was $ 6.8 million and $ 4.3 million respectively, in unrecognized compensation cost related to RSUs expected to be recognized over a remaining weighted-average period of 2.5 years and 2.6 years, respectively. The total fair value of RSUs that vested during 2023 and 2022 was $ 1.7 million and $ 0.6 million, respectively. RSAs The following table summarizes the activity related to RSAs during 2023 and 2022 (shares in thousands): Number of Shares Weighted-Average Grant Date Fair Value Outstanding unvested as of December 31, 2021 89 $ 3.87 Vested ( 46 ) $ 3.87 Forfeited ( 28 ) $ 3.87 Outstanding unvested as of December 31, 2022 15 $ 3.87 Vested ( 14 ) $ 3.87 Forfeited ( 1 ) $ 3.87 Outstanding unvested as of December 31, 2023 — $ — As of December 31, 2023 , there was no remaining unrecognized compensation cost related to RSAs. As of December 31, 2022 , there was $ 0.1 million in unrecognized compensation cost related to RSAs expected to be recognized over a remaining weighted-average period of 1.0 year. The total fair value of RSAs that vested during 2023 and 2022 was $ 0.1 million and $ 0.6 million, respectively. 2021 Employee Stock Purchase Plan The 2021 Employee Stock Purchase Plan (the "ESPP") authorizes the issuance of shares of common stock to participating eligible employees and provides for two six-month offering periods each year. As of December 31, 2023 , there were approximately 1.3 million shares remaining for future purchases under the ESPP. The ESPP provides that the number of shares reserved and available for issuance under the ESPP will automatically increase each January 1 by the lesser of 0.8 million shares of the Company's common stock, 1 % of the outstanding number of shares of Cullinan’s common stock on the immediately preceding December 31, or such lesser number of shares as determined by the Company’s compensation committee. On January 1, 2024, the total number of shares available for issuance under the ESPP increased by approximately 0.4 million shares under this provision. During each of 2023 and 2022 , Cullinan issued less than 0.1 million shares of its common stock pursuant the ESPP. Subsidiary Stock Options Prior to being acquired by the Company, the previous board of directors of the Company's CLN-619 development subsidiary authorized the grant of stock options to employees, directors of, consultants and other key persons to the entity. In October 2022, the current board of directors of Cullinan's CLN-619 development subsidiary authorized the acceleration of vesting for approximately 0.3 million stock options. The vesting acceleration was determined to be a cancellation of the prior award with a concurrent grant of a replacement award and was accounted for as a modification resulting in $ 0.6 million in incremental equity-based compensation expense. As of December 31, 2023, the Company's CLN-619 development subsidiary h ad approximately 0.2 million stock options held by noncontrolling interests that were outstanding and exercisable with a weighted-average exercise price of $ 0.22 per share.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2023
Related Party Transactions [Abstract]
Related Party Transactions	(10) Related Party Transactions The Company's CLN-619, CLN-049 and CLN-617 development subsidiaries are each party to royalty transfer agreements with two charitable foundations that are affiliated with an investor which beneficially owns more than five percent of the Company’s outstanding common stock. Under these royalty transfer agreements, the charitable foundations are entitled to receive a low single digit royalty percentage of all global net sales of any products developed by the applicable 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. Cullinan has concluded that these instruments had no value at the inception of the agreements. Given the early-stage nature of the underlying technologies and 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, 2023 and December 31, 2022 . Cullinan currently does not have any applicable net sales from its products and as a result, has no t paid or incurred any royalties under these agreements as of December 31, 2023 . The Company will monitor these instruments for changes in fair value at each reporting date.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	(11) Income Taxes During the fourth quarter of 2023, Cullinan recorded a current income tax benefit of $ 14.1 million . The income tax benefit recorded for 2023 was driven by return to provision adjustments for the utilization of 2022 and historical tax attributes against the gain on sale of Cullinan Pearl and the expected utilization of federal research and development credits generated during 2023 that can be carried back to 2022. During 2022 , the Company recorded a current income tax expense of $ 42.1 million. The income tax expense recorded for 2022 was driven by the tax from the gain on sale of Cullinan Pearl, partially offset by the expected utilization of tax attributes generated during 2022 and the release of valuation allowance for the expected utilization of certain historical tax attributes against the gain from the sale. Refer to Note 3 for additional details on the sale of Cullinan Pearl. The Company’s net income (loss) before income taxes consists solely of domestic income and losses in each of 2023 and 2022. As of December 31, 2023 , Cullinan had recorded $ 5.4 million within prepaid expenses and other current assets on its consolidated balance sheets, which was comprised of $ 3.9 million for 2022 state tax refund claims and $ 1.5 million for the expected carryback of 2023 federal research and development credits, partially offset by a refund payment from the Internal Revenue Service (the "IRS") that was in excess of the Company’s 2022 federal refund claim. A reconciliation of the Company’s statutory income tax rate to its effective income tax rate in 2023 and 2022 is as follows: 2023 2022 Federal statutory rate 21.00 % 21.00 % State taxes, net of federal benefit 6.70 % 4.71 % Research and development credits 6.28 % ( 0.25 )% Equity-based compensation ( 1.49 )% 0.77 % Valuation allowance ( 24.41 )% 1.13 % Other, net 0.27 % 0.45 % Effective tax rate 8.35 % 27.81 % As of December 31, 2023 and 2022, the net deferred income tax asset balance related to the following (in thousands): December 31, 2023 2022 Deferred tax assets: Capitalized research and development $ 41,901 $ 18,724 Equity-based compensation 16,960 12,087 Net operating loss 14,443 10,531 Licenses 7,189 461 Research and development credit 3,155 1,110 Accrued expenses 1,836 1,977 Basis difference on gain on sale of Cullinan Pearl 1,700 1,805 Lease liability 985 1,345 Capitalized organizational and start-up expenses 116 127 Gross deferred tax assets 88,285 48,167 Valuation allowance ( 87,371 ) ( 46,766 ) Net deferred tax asset 914 1,401 Deferred tax liability ROU asset 697 1,108 Depreciation and amortization 217 293 Net deferred tax asset $ — $ — The Company's net operating loss ("NOL") and tax credit carryforwards are subject to review and possible adjustment by the IRS and state tax authorities. Under Sections 382 and 383 of the Internal Revenue Code of 1986, as amended, as well as similar state provisions, NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company. As of December 31, 2023 and 2022 , the Company had federal NOL carryforwards, of $ 54.3 million and $ 42.6 million, respectively, which may be available to offset future income tax liabilities. As of December 31, 2023 , $ 52.9 million of Cullinan's federal NOL carryforwards can be carried forward indefinitely, and the remaining $ 1.4 million expires in 2037 . As of December 31, 2023 and 2022 , the Company had state NOL carryforwards of $ 56.8 million and $ 27.1 million, respectively, which may be available to offset future income tax liabilities. As of December 31, 2023 , Cullinan's state NOL carryforwards begin to expire in 2031 . As of December 31, 2023 and 2022 , the Company had federal research and development tax credit carryforwards of $ 1.6 million and $ 0.9 million, respectively. As of December 31, 2023 , Cullinan’s federal research and development tax credit carryforwards begin to expire in 2036 . As of December 31, 2023 and 2022 , the Company had state research and development tax credit carryforwards of $ 2.0 million and $ 0.3 million, respectively. As of December 31, 2023, $ 0.3 million of Cullinan's state research and development tax credit carryforwards can be carried forward indefinitely, and the remaining $ 1.7 million expires beginning in 2036 . Cullinan has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets, which primarily consist of capitalized research and development costs, temporary differences on equity-based compensation, and NOL carryforwards. The Company has considered its history of cumulative net losses, with the exception of the one-time gain on the sale of Cullinan Pearl in 2022, estimated future taxable income and prudent and feasible tax planning strategies and has concluded that it is more likely than not that Cullinan will not realize the benefits of its deferred tax assets. As a result, as of December 31, 2023, the Company has maintained a full valuation allowance against its remaining net deferred tax assets. Cullinan’s valuation allowance increased during 2023 and 2022 primarily due to capitalized research and development costs and the generation of NOLs as follows (in thousands): 2023 2022 Valuation allowance at beginning of year $ 46,766 $ 44,552 Increases recorded to income tax provision 41,307 1,712 Increases (decreases) recorded to equity ( 702 ) 502 Valuation allowance at end of year $ 87,371 $ 46,766 The calculation of the Company’s tax liabilities involves dealing with uncertainties in the application of complex tax laws and regulations for both federal taxes and the states in which Cullinan operates or does business in. The Company recognizes a tax benefit from an uncertain tax position when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, on the basis of the technical merits. Cullinan records uncertain tax positions as liabilities and adjusts these liabilities when its judgment changes as a result of the evaluation of new information not previously available. Due to the complexity of some of these uncertainties, the ultimate resolution may result in a payment that is materially different from the Company’s current estimate of the unrecognized tax benefit liabilities. These differences will be reflected as increases or decreases to income tax expense in the period in which new information is available. As of December 31, 2023 and 2022 , Cullinan has no t recorded a liability for any uncertain tax positions in its consolidated financial statements. The Company recognizes interest and penalties related to unrecognized tax benefits on the income tax expense line in the accompanying consolidated statements of operations and comprehensive income (loss). As of December 31, 2023 and 2022 , no accrued interest or penalties are included in the consolidated balance sheets. Cullinan files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions in the U.S. There are currently no pending tax examinations. Cullinan's federal and state income tax returns are generally subject to tax examinations for tax years 2016 and later . To the extent that the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the IRS and the state tax authorities to the extent utilized in a future period.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	(12) Commitments and Contingencies Cullinan enters into contracts in the normal course of business with contract research organizations, contract manufacturing organizations, and other third parties for preclinical research studies, clinical trials and testing and manufacturing services. These agreements generally include cancellation clauses. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, Cullinan has entered into indemnification agreements with members of its board of directors and executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments Cullinan could be required to make under these indemnification agreements is, in certain cases, unlimited. To date, Cullinan has not incurred any material costs as a result of such indemnifications. Cullinan is not aware of any indemnification arrangements that could have a material effect on its financial position, results of operations or cash flows, and it has no t accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2023 and 2022. Legal Proceedings The Company is not currently party to or aware of any material legal proceedings. At each reporting date, Cullinan evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Leases

Leases	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Leases	(13) Leases Cullinan has an operating lease for approximately 8,000 square feet of office space in a multi-tenant building in Cambridge, Massachusetts, which commenced in February 2018 and goes through June 2024 (the "Suite 520 Lease"). In August 2022, the Company entered into an additional operating lease (the "Suite 1350 Lease") for approximately 14,000 square feet of office space in a multi-tenant building in Cambridge, Massachusetts through July 2026. Lease expense consisted of operating lease costs of $ 1.7 million and $ 1.1 million for 2023 and 2022, respectively. The following table summarizes supplemental cash flow information for 2023 and 2022 (in thousands): 2023 2022 Cash paid for amounts included in measurement of lease liabilities: Operating cash flows from operating leases (1) $ 1,881 $ 260 ROU asset obtained in exchange for an operating lease liability $ — $ 4,931 (1) Operating cash flows from operating leases for 2022 includes cash inflow of $ 0.3 million reimbursed by the lessor for improvements made to the newly leased office space pursuant to the terms of the Suite 1350 lease. The following table summarizes the weighted-average lease term and discount rate as of December 31, 2023 and 2022: 2023 2022 Weighted-average remaining lease term (in years) 2.4 3.2 Weighted-average discount rate 10.9 % 10.8 % As Cullinan’s operating leases did not provide an implicit rate, the Company used its incremental borrowing rate based on the information available in determining the present value of lease payments. Cullinan’s incremental borrowing rate was based on the term of the lease, the economic environment and reflects the rate the Company would have had to pay to borrow on a secured basis. The following table summarizes Cullinan’s future minimum lease payments as of December 31, 2023 (in thousands): December 31, 2023 2024 $ 1,738 2025 1,461 2026 872 Total future minimum lease payments 4,071 Less: imputed interest ( 481 ) Total lease liabilities at present value $ 3,590 Sublease Agreement In September 2022, Cullinan entered into a sublease agreement through May 2024 for the office space that the Company leases under the Suite 520 Lease. For 2023 and 2022 , Cullinan recorded sublease income of $ 0.5 million and $ 0.1 million, respectively, within other income (expense), net. In September 2023, Cullinan and its subtenant cancelled the sublease agreement for the office space that the Company leases under the Suite 520 Lease, and Cullinan determined that the remaining right-of-use asset for the Suite 520 Lease and the related leasehold improvements (“Suite 520 Asset Group”) was not recoverable. Upon determining that the remaining Suite 520 Asset Group was not recoverable, Cullinan recorded an impairment of long-lived assets of $ 0.4 million for the carrying value in excess of the fair value of the Suite 520 Asset Group within income (loss) from operations in its consolidated statements of operations and other comprehensive income (loss) for 2023 .

Net Income (Loss) per Share Att

Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan	(14) Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan The following table sets forth the calculation of basic and diluted net income (loss) per share attributable to common stockholders of Cullinan for 2023 and 2022 (in thousands, except per share data): 2023 2022 Numerator: Net income (loss) attributable to common stockholders of Cullinan $ ( 153,162 ) $ 111,214 Denominator: Weighted-average common stock outstanding - basic 41,550 45,164 Dilutive effect of common stock issuable from assumed exercise of equity awards — 1,476 Weighted-average common stock outstanding - diluted 41,550 46,640 Net income (loss) per share attributable to common stockholders of Cullinan: Basic $ ( 3.69 ) $ 2.46 Diluted $ ( 3.69 ) $ 2.38 The Company used the treasury stock method for equity awards, and the if-converted method for preferred stock, to determine the number of dilutive shares. The following table sets forth potential common shares that were excluded from the computation of the diluted net income (loss) per share attributable to common stockholders of Cullinan for 2023 and 2022 because their effect would have been anti-dilutive (in thousands): 2023 2022 Stock options 9,418 6,842 Preferred stock 6,138 — RSAs and RSUs 147 25 ESPP 9 7 Total 15,712 6,874

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and in accordance with applicable rules and regulations of the Securities and Exchange Commission ("SEC") for financial reporting and include accounts of the Company and its consolidated subsidiaries.
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"). Under the VOE, the Company consolidates the entity if it determines 1) that it directly, or indirectly, has greater than 50% of the voting shares or other equity holders do not have substantive voting, participation, or liquidation rights, or 2) when the company has a controlling financial interest through its control of the board of directors, and the significant decisions of the entity are made at the board level. The Company concluded that none of its subsidiaries is a VIE and has consolidated each of its subsidiaries under the VOE. Intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of Cullinan’s consolidated financial statements and accompanying notes in conformity with U.S. GAAP requires the Company’s management to make estimates and judgments that affect the amounts reported in the financial statements. On an ongoing basis, Cullinan’s management evaluates its estimates, which include, but are not limited to, estimates related to prepaid and accrued research and development expenses, equity-based compensation, income taxes, and the fair value of royalty transfer agreements. Management's estimates could change period to period based on changes in facts and circumstances. The Company’s management bases its estimates on historical experience and on other relevant assumptions that are believed to be reasonable. Actual results may differ materially from these estimates.
Segments	Segments Cullinan has determined that its Chief Executive Officer is the Chief Operating Decision Maker ("CODM"). The Company operates and manages the business as one reporting and one operating segment, which is the business of developing early-stage cancer therapeutics. Cullinan’s CODM reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets were located in the U.S. as of each of December 31, 2023 and 2022. Expenditures for additions to long-lived assets included purchases of property and equipment in each of 2023 and 2022 .
Concentration of Risk	Concentration of Risk Cullinan had no significant concentration of credit risk as of December 31, 2023. Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. as of December 31, 2023. Deposits at banks may exceed the insurance provided on such deposits. These deposits may be redeemed upon demand, and therefore, bear minimal risk. Under our investment policy, the Company limits amounts invested in such securities by investment type, credit rating, maturity, industry group and issuer. The goals of our investment policy are (i) safety and preservation of principal and diversification of risk and (ii) liquidity of investments sufficient to meet cash flow requirements. Cullinan is subject to certain risks and uncertainties and believes that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: ability to conduct and complete preclinical and clinical trials of our current and future product candidates; ability to obtain future financing; ability to build a successful pipeline of product candidates, including efficient expenditures of its resources; regulatory approval and market acceptance of, and reimbursement for, current and future product candidates; protection of Cullinan’s intellectual property, including litigation or claims against the Company based on intellectual property, patent, product, regulatory or other factors; performance of third-party clinical research organizations and manufacturers upon which the Company relies; and Cullinan’s ability to attract and retain employees necessary to support its growth. The Company is dependent and expects to continue to be dependent on a small number of third-party manufacturers to supply drug product and drug substance for research and development activities in its programs. These programs could be adversely affected by a significant interruption in supply.
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 each of December 31, 2023 and 2022 , cash equivalents consist of government-backed money market funds.
Investments	Investments Cullinan generally holds investments in marketable securities. Investments not classified as cash equivalents with maturities of less than twelve months are classified as short-term investments in the consolidated balance sheets. Investments 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. The amortized cost of marketable securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion is included in interest income. Dividends are also included in interest income. Interest receivable is included in prepaid expenses and other current assets on the consolidated balance sheets and represents accrued and unpaid interest on Cullinan's marketable securities. The Company periodically reviews its marketable securities for impairment and adjusts these investments to their fair value when a decline in market value is deemed to be other than temporary. Declines in fair value judged to be other-than-temporary on marketable securities, if any, are included in other income (expense), net.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Cullinan has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements. The three levels of the fair value hierarchy are described below: Level 1—Unadjusted quoted prices for identical assets or liabilities in active markets that the Company has the ability to access; Level 2—Quoted prices for similar assets and liabilities in active markets or other market-observable inputs such as interest rates, yield curves and foreign currency spot rates; and Level 3—Pricing or valuations that require inputs that are both significant to the fair value measurement and unobservable. There were no transfers of financial assets or liabilities measured at fair value between Level 1 and Level 2, and there were no Level 3 investments during 2023 or 2022. Cullinan's financial assets recorded at fair value consist of investments. The fair value of the Company’s investments is primarily determined using market quotations or prices obtained from independent pricing sources. As of December 31, 2023 and 2022 , the fair values of cash and cash equivalents, prepaid expenses and other current assets, accounts payable, accrued expenses and other current liabilities approximated their carrying values due to the short-term nature of these instruments
Property and Equipment, net	Property and Equipment, net Property and equipment is stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Asset Class Estimated Useful Life Office furniture and equipment 5 years Leasehold improvements Shorter of the useful life of the asset or the lease term Maintenance and repairs that do not improve or extend the life of the assets are expensed when incurred. Upon disposal or retirement of assets, the cost and accumulated depreciation and amortization are removed from the consolidated balance sheets and any resulting gain or loss is reflected in the consolidated statements of operations and comprehensive income (loss).
Leases	Leases Cullinan determines if an arrangement is a lease at inception. A contract is or contains a lease if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company classifies leases at the lease commencement date as operating or finance leases and records a right-of-use asset ("ROU") and a lease liability on the consolidated balance sheets for all leases with an initial lease term of greater than 12 months. Leases with an initial term of 12 months or less are not recorded in the balance sheet, and payments are recognized as expense on a straight-line basis over the lease term. Cullinan enters into contracts that contain both lease and non-lease components. Non-lease components may include maintenance, utilities and other operating costs. The Company combines the lease and non-lease components of fixed costs in its lease arrangements as a single lease component. Variable costs, such as utilities or maintenance costs, are not included in the measurement of ROU assets and lease liabilities but rather are expensed when the event determining the amount of variable consideration to be paid occurs. Operating lease assets and liabilities are recognized at the lease commencement date based on the present value of the lease payments over the lease term using the discount rate implicit in the lease. If the discount rate is not readily determinable, Cullinan utilizes an estimate of its incremental borrowing rate based upon the available information at the lease commencement date. Operating lease assets are further adjusted for prepaid or accrued lease payments. Operating lease payments are expensed using the straight-line method as an operating expense over the lease term. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that Cullinan will exercise that option.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to future undiscounted net cash flows expected to be generated. Impairment charges are recognized at the amount by which the carrying amount of an asset exceeds the fair value of the asset. Assets to be disposed of are reported at the lower of the carrying amount or the fair value less costs to sell.
Noncontrolling Interests	Noncontrolling Interests Noncontrolling interests represent third-party interests in the Company’s subsidiaries. Cullinan determines the amount of the noncontrolling interests in the net assets of the Company’s subsidiaries at each balance sheet date using the hypothetical liquidation at book value ("HLBV") method. Under the HLBV method, the amounts reported as noncontrolling interests in the consolidated balance sheets represent the amounts third parties would hypothetically receive at each balance sheet date under the liquidation provisions of the subsidiaries, assuming the net assets of the subsidiaries were liquidated at their recorded amounts determined in accordance with U.S. GAAP and distributed to the owners of the subsidiaries. Net income (loss) attributable to noncontrolling interests on the consolidated statements of operations and comprehensive income (loss) is determined as the difference in the noncontrolling interest in the consolidated balance sheets between the start and end of each reporting period, after taking into account any capital transactions between the subsidiaries and third parties.
Revenue Recognition	Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services. The amount of revenue recognized reflects the consideration to which Cullinan expects to be entitled in exchange for these goods and services. To achieve this core principle, the Company applies the following five steps: 1) identify the customer contract; 2) identify the contract’s performance obligations; 3) determine the transaction price; 4) allocate the transaction price to the performance obligations; and 5) recognize revenue when or as a performance obligation is satisfied. Licensing arrangements are analyzed to determine whether the promised goods or services, which could include licenses and research and development materials and services, are distinct or whether they must be accounted for as part of a combined performance obligation. The transaction price is determined based on the consideration to which Cullinan will be entitled. The transaction price may include fixed amounts, variable amounts, or both. The Company reevaluates the probability of realizing such variable consideration and any related constraints at each reporting period. Cullinan includes variable consideration in the transaction price to the extent it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur. The Company allocates the transaction price based on the estimated standalone selling price of the underlying performance obligations. Cullinan must develop assumptions that require judgment to determine the standalone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the standalone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs to complete the respective performance obligation. Cullinan also utilizes judgment in assessing whether or not variable consideration is constrained or if it can be allocated specifically to one or more performance obligations in the arrangement. When a performance obligation is satisfied, revenue is recognized for the amount of the transaction price allocated to that performance obligation on a relative standalone selling price basis, which excludes estimates of variable consideration that are constrained. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. For performance obligations consisting of licenses and other promises, Cullinan utilizes judgment to assess whether the combined performance obligation is satisfied over time or at a point in time and the recognition pattern for the portion of the transaction price allocated to the performance obligation.
Research and Development Expenses	Research and Development Expenses Research and development costs are expensed as incurred. Research and development expenses consist primarily of employee compensation costs and amounts incurred with 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 Cullinan 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. Cullinan has entered into various research and development related contracts with parties both inside and outside of the U.S. The payments related to these agreements are recorded as research and development expenses as incurred. The Company records accrued liabilities for estimated ongoing research and development costs. When evaluating the adequacy of the accrued liabilities, Cullinan 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.
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 using the fair value of the awards and is recognized as expense over the requisite service period, which is generally the vesting period. Forfeitures are recognized as they occur. Cullinan 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. The fair value of service-based restricted stock units ("RSUs") is the closing market price of the Company's common stock on the grant date. The fair value of market-based RSUs is measured on the grant date using a Monte Carlo simulation model. Cullinan estimated the fair value of stock options using the Black-Scholes option pricing model. Both the Monte Carlo simulation model and the Black-Scholes option pricing model require the input of objective and subjective assumptions. Certain assumptions used, including the Company’s expected stock price volatility, 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. Prior to 2023, the expected volatility used in the Black-Scholes option pricing model for new options was based on historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption. In 2023, Cullinan determined that a sufficient amount of historical information was available regarding the volatility of its stock price to begin using a blended rate that combines the Company's historical volatility with the historical volatilities of the stock prices of similar entities within the Company’s industry over a period of time commensurate with the expected term assumption.
Income Taxes	Income Taxes Cullinan recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount of benefit that is greater than fifty percent likely to be realized upon settlement. Changes in measurement are reflect in the period in which the change in judgment occurs.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from nonowner sources. The Company’s only element of other comprehensive loss is unrealized gains and losses on investments.
Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan	Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan Basic net income (loss) per share attributable to common stockholders of Cullinan is determined by dividing net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period. Diluted net income (loss) per share attributable to common stockholders of Cullinan is determined by dividing earnings (net loss) attributable to common stockholders by the weighted-average number of common shares outstanding during the period, adjusted for the dilutive effect of shares of common stock equivalents as determined using the treasury stock method for equity awards and the if-converted method for preferred stock.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the "JOBS Act"). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. Cullinan has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these consolidated financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. The JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies. The Company expects to use the extended transition period for any other new or revised accounting standards during the period in which it remains an emerging growth company.
Recently Adopted Accounting Pronouncements	Recently Issued Accounting Pronouncements In November 2023, the Financial Accounting Standards Board (the “FASB”) issued an accounting standards update to improve reportable segment disclosure requirements, primarily through enhanced disclosures about significant segment expenses. The main provisions of this update require companies to disclose, on an annual and interim basis, significant segment expenses, segment profit and loss, and other segments items that are regularly provided to the CODM. This update also requires companies to disclose the title and position of the CODM and to explain how the CODM uses the reported segment measures in assessing segment performance and deciding how to allocate resources. The update also requires companies with a single reportable segment to provide all required segment reporting disclosures. This new standard will be effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted. Cullinan adopted this standard on January 1, 2024 for 2024 annual reporting and interim periods beginning in 2025. This new standard will not have a material impact on the Company’s consolidated financial statements and associated disclosures. In December 2023, the FASB issued an accounting standards update to enhance transparency about income tax information through improvements to income tax disclosures primarily related to the rate reconciliation and income taxes paid information. The main provisions in this update will require companies to disclose, on an annual basis, specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. This update will also require companies to disclose, on an annual basis, the amount of income taxes paid, income (or loss) from continuing operations before income tax expense (or benefit), and income tax expense (or benefit) from continuing operations, disaggregated between federal, state, foreign, and jurisdictional taxes. This new standard will be effective beginning for fiscal years beginning after December 15, 2025, and early adoption is permitted. The Company expects that it will adopt this new standard on January 1, 2026. The Company is evaluating the impact this new standard will have on its consolidated financial statements and associated disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Estimated Useful Life of Property and Equipment	Property and equipment is stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Asset Class Estimated Useful Life Office furniture and equipment 5 years Leasehold improvements Shorter of the useful life of the asset or the lease term

Sale of Cullinan Pearl (Tables)

Sale of Cullinan Pearl (Tables)	12 Months Ended
	Dec. 31, 2023
Debt Securities, Available-for-Sale [Abstract]
Schedule of cash consideration received	The table below sets forth the book value of the Cullinan Pearl assets and liabilities sold along with the calculation of the gain on sale based on the cash consideration received (in thousands): Book value of assets sold Cash $ 2,898 Prepaid expenses and other current assets 619 Amounts attributable to assets sold 3,517 Book value of liabilities sold Accrued expenses and other current liabilities 2,404 Amounts attributable to liabilities sold 2,404 Total identifiable net assets sold 1,113 Upfront consideration, inclusive of cash transferred of $ 2,898 277,898 Gain on sale of Cullinan Pearl $ 276,785

Financial Instruments (Tables)

Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2023
Financial Instruments Pledged as Collateral [Abstract]
Schedule of Short-term and Long-term Investment Marketable Securities by Security Type	Cullinan recognized its investments by security type at December 31, 2023 as follows (in thousands): Amortized Gross Gross Estimated Short-term investments U.S. government notes $ 208,289 $ 221 $ ( 16 ) $ 208,494 Corporate notes 99,359 27 ( 275 ) 99,111 Asset-backed securities 61,114 3 ( 89 ) 61,028 Total short-term investments 368,762 251 ( 380 ) 368,633 Total investments $ 368,762 $ 251 $ ( 380 ) $ 368,633 Cullinan recognized its investments by security type at December 31, 2022 as follows (in thousands): Amortized Gross Gross Estimated Short-term investments Corporate notes $ 244,498 $ 11 $ ( 1,743 ) $ 242,766 U.S. government notes 34,029 — ( 290 ) 33,739 Commercial paper 18,035 3 ( 13 ) 18,025 Asset-backed securities 16,625 — ( 15 ) 16,610 Total short-term investments 313,187 14 ( 2,061 ) 311,140 Long-term investments Corporate notes 81,436 18 ( 572 ) 80,882 Total long-term investments 81,436 18 ( 572 ) 80,882 Total investments $ 394,623 $ 32 $ ( 2,633 ) $ 392,022
Summary of Fair Value of Financial Assets Measured on Recurring Basis	The following table sets forth the fair value of Cullinan’s financial assets that were measured at fair value on a recurring basis as of December 31, 2023 (in thousands): Level 1 Level 2 Level 3 Total Short-term investments U.S. government notes $ — $ 208,494 $ — $ 208,494 Corporate notes — 99,111 — 99,111 Asset-backed securities — 61,028 — 61,028 Total short-term investments — 368,633 — 368,633 Total investments $ — $ 368,633 $ — $ 368,633 The following table sets forth the fair value of Cullinan’s financial assets that were measured at fair value on a recurring basis as of December 31, 2022 (in thousands): Level 1 Level 2 Level 3 Total Short-term investments Corporate notes $ — $ 242,766 $ — $ 242,766 U.S. government notes — 33,739 — 33,739 Commercial paper — 18,025 — 18,025 Asset-backed securities — 16,610 — 16,610 Total short-term investments — 311,140 — 311,140 Long-term investments Corporate notes — 80,882 — 80,882 Total long-term investments — 80,882 — 80,882 Total investments $ — $ 392,022 $ — $ 392,022

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment	Property and equipment, net consisted of the following as of December 31, 2023 and 2022 (in thousands): December 31, 2023 2022 Office furniture and equipment $ 765 $ 681 Leasehold improvements 576 628 Total property and equipment, gross 1,341 1,309 Less: accumulated depreciation ( 352 ) ( 135 ) Total property and equipment, net $ 989 $ 1,174

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2023
Accrued Expenses And Other Current Liabilities [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following as of December 31, 2023 and 2022 (in thousands): December 31, 2023 2022 Contracted research and development expenses $ 8,434 $ 7,486 Due to Taiho under collaboration agreement, net 7,869 — Employee compensation 6,987 4,516 Other current liabilities 914 1,955 Convertible note and accrued interest — 178 $ 24,204 $ 14,135

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2023
Stockholders' Equity Note [Abstract]
Schedule of development subsidiaries and their product candidates	The following table shows the Company’s ownership interest as of December 31, 2023 in its development subsidiaries and their product candidates: Development Subsidiary (Product Candidate) Ownership as of Cullinan MICA Corp. (CLN-619) 95 % Cullinan Florentine Corp. (CLN-049) 96 % Cullinan Amber Corp. (CLN-617) 94 %

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Schedule of Equity-based Compensation Expense Categories In Consolidated Statements of Operations and Comprehensive Loss	The Company recorded equity-based compensation in the following expense categories in the consolidated statements of operations and comprehensive income (loss) in 2023 and 2022 (in thousands): 2023 2022 General and administrative $ 18,270 $ 16,939 Research and development 12,168 11,018 Total equity-based compensation $ 30,438 $ 27,957
Schedule of Assumptions Used to Estimate Fair value	The grant date fair value was estimated at the time of grant using the Black-Scholes option-pricing model using the following weighted-average assumptions in 2023 and 2022: 2023 2022 Risk-free interest rate 4.0 % 2.7 % Expected term (in years) 6.0 6.0 Expected volatility 78.7 % 79.8 % Expected dividend yield 0.0 % 0.0 %
Summary of Stock Options Activity	The following table summarizes 2023 and 2022 stock option activity (options and aggregate intrinsic value in thousands): Number of Options Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value Outstanding as of December 31, 2021 9,259 $ 15.77 Granted 2,888 $ 12.96 Exercised ( 1,523 ) $ 4.31 Forfeited ( 1,290 ) $ 13.80 Outstanding as of December 31, 2022 9,334 $ 17.04 Granted 2,091 $ 10.96 Exercised ( 58 ) $ 4.30 Forfeited ( 828 ) $ 22.63 Outstanding as of December 31, 2023 10,539 $ 15.46 7.94 $ 10,753 Exercisable as of December 31, 2023 5,535 $ 15.37 7.49 $ 10,170
Summary of Activity Related to RSUs	Number of Shares Weighted-Average Grant Date Fair Value Outstanding unvested as of December 31, 2021 — $ — Granted (1) 502 $ 13.28 Vested ( 46 ) $ 13.60 Forfeited ( 87 ) $ 13.60 Outstanding unvested as of December 31, 2022 369 $ 13.16 Granted 638 $ 11.29 Vested ( 178 ) $ 11.90 Forfeited ( 25 ) $ 12.26 Outstanding unvested as of December 31, 2023 804 $ 11.98 (1) The number granted represents the number of shares issuable upon vesting of service-based and market-based RSUs, assuming the Company achieves its corporate stock price metrics at the target achievement level.
Summary of Activity Related to RSAs	The following table summarizes the activity related to RSAs during 2023 and 2022 (shares in thousands): Number of Shares Weighted-Average Grant Date Fair Value Outstanding unvested as of December 31, 2021 89 $ 3.87 Vested ( 46 ) $ 3.87 Forfeited ( 28 ) $ 3.87 Outstanding unvested as of December 31, 2022 15 $ 3.87 Vested ( 14 ) $ 3.87 Forfeited ( 1 ) $ 3.87 Outstanding unvested as of December 31, 2023 — $ —
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Schedule of Assumptions Used to Estimate Fair value	The following table details the assumptions used in the Monte Carlo simulation model used to estimate the fair value of the market-based RSUs granted during 2022: 2022 Stock price $ 12.98 Volatility 82.5 % Expected term (in years) 2.7 Risk-free rate 2.9 % Expected dividend yield 0.0 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Summary of Reconciliation of Statutory Income Tax Rate To Effective Income Tax Rate	A reconciliation of the Company’s statutory income tax rate to its effective income tax rate in 2023 and 2022 is as follows: 2023 2022 Federal statutory rate 21.00 % 21.00 % State taxes, net of federal benefit 6.70 % 4.71 % Research and development credits 6.28 % ( 0.25 )% Equity-based compensation ( 1.49 )% 0.77 % Valuation allowance ( 24.41 )% 1.13 % Other, net 0.27 % 0.45 % Effective tax rate 8.35 % 27.81 %
Summary of Net Deferred Income Tax Asset	As of December 31, 2023 and 2022, the net deferred income tax asset balance related to the following (in thousands): December 31, 2023 2022 Deferred tax assets: Capitalized research and development $ 41,901 $ 18,724 Equity-based compensation 16,960 12,087 Net operating loss 14,443 10,531 Licenses 7,189 461 Research and development credit 3,155 1,110 Accrued expenses 1,836 1,977 Basis difference on gain on sale of Cullinan Pearl 1,700 1,805 Lease liability 985 1,345 Capitalized organizational and start-up expenses 116 127 Gross deferred tax assets 88,285 48,167 Valuation allowance ( 87,371 ) ( 46,766 ) Net deferred tax asset 914 1,401 Deferred tax liability ROU asset 697 1,108 Depreciation and amortization 217 293 Net deferred tax asset $ — $ —
Summary of Valuation Allowance	2023 2022 Valuation allowance at beginning of year $ 46,766 $ 44,552 Increases recorded to income tax provision 41,307 1,712 Increases (decreases) recorded to equity ( 702 ) 502 Valuation allowance at end of year $ 87,371 $ 46,766

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Supplemental Cash Flow Information Relating to Leases	The following table summarizes supplemental cash flow information for 2023 and 2022 (in thousands): 2023 2022 Cash paid for amounts included in measurement of lease liabilities: Operating cash flows from operating leases (1) $ 1,881 $ 260 ROU asset obtained in exchange for an operating lease liability $ — $ 4,931 Operating cash flows from operating leases for 2022 includes cash inflow of $ 0.3 million reimbursed by the lessor for improvements made to the newly leased office space pursuant to the terms of the Suite 1350 lease.
Summarizes Lease Term and Discount Rate	The following table summarizes the weighted-average lease term and discount rate as of December 31, 2023 and 2022: 2023 2022 Weighted-average remaining lease term (in years) 2.4 3.2 Weighted-average discount rate 10.9 % 10.8 %
Summarizes of future minimum lease payments	The following table summarizes Cullinan’s future minimum lease payments as of December 31, 2023 (in thousands): December 31, 2023 2024 $ 1,738 2025 1,461 2026 872 Total future minimum lease payments 4,071 Less: imputed interest ( 481 ) Total lease liabilities at present value $ 3,590

Net Income (Loss) per Share A_2

Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan (Tables)	12 Months Ended
	Dec. 31, 2023
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 income (loss) per share attributable to common stockholders of Cullinan for 2023 and 2022 (in thousands, except per share data): 2023 2022 Numerator: Net income (loss) attributable to common stockholders of Cullinan $ ( 153,162 ) $ 111,214 Denominator: Weighted-average common stock outstanding - basic 41,550 45,164 Dilutive effect of common stock issuable from assumed exercise of equity awards — 1,476 Weighted-average common stock outstanding - diluted 41,550 46,640 Net income (loss) per share attributable to common stockholders of Cullinan: Basic $ ( 3.69 ) $ 2.46 Diluted $ ( 3.69 ) $ 2.38
Summary of Equity Instruments Excluded from Computation of Diluted Net Loss Per Share	The following table sets forth potential common shares that were excluded from the computation of the diluted net income (loss) per share attributable to common stockholders of Cullinan for 2023 and 2022 because their effect would have been anti-dilutive (in thousands): 2023 2022 Stock options 9,418 6,842 Preferred stock 6,138 — RSAs and RSUs 147 25 ESPP 9 7 Total 15,712 6,874

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) $ in Millions	Dec. 31, 2023 USD ($)
Nature Of Business And Basis Of Presentation [Line Items]
Cash equivalents short term investments and interest recivable	$ 468.3

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) Segment	Dec. 31, 2022 USD ($)
Accounting Policies [Line Items]
Description of 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.
Unrealized gain (loss) on investments | $	$ 2,472	$ (1,763)
Number of reporting segments	1
Number of operating segments	1
Operating Lease Term	12 months

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Estimated Useful Life of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2023
Office Furniture and Equipment
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property Plant And Equipment Dispositions	Shorter of the useful life of the asset or the lease term

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of balance sheet adjustments in connection with ASC 842 (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Operating lease right-of-use assets	$ 2,543	$ 4,130
Operating lease liabilities, current	1,440	1,421
Lease liabilities, non-current	$ 2,150	$ 3,590

Sale of Cullinan Pearl (Additio

Sale of Cullinan Pearl (Additional Information) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2022	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Cash transferred with sale		$ 2,898
Gain on sale of Cullinan Pearl		0	$ 276,785
Repayment of convertible note		0	2,200
Research and development costs		148,156	91,948
Prepaid expenses and other current assets		619
Pearl - Taiho
Defined Benefit Plan Disclosure [Line Items]
Receive upfront payment from party	$ 275,000
Cash transferred with sale	2,900
Additional receive payments from the party	$ 130,000
Gain on sale of Cullinan Pearl		276,800
Repayment of convertible note		2,200
Percentage of profits from potential sales	50%
Research and development costs		6,900	3,500
Research and development expense related to share of costs		$ 25,300	$ 2,500

Sale of Cullinan Pearl - Schedu

Sale of Cullinan Pearl - Schedule of cash consideration received (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Book value of assets sold
Cash	$ 2,898
Prepaid expenses and other current assets	619
Amounts attributable to assets sold	3,517
Book value of liabilities sold
Accrued expenses and other current liabilities	2,404
Amounts attributable to liabilities sold	2,404
Total identifiable net assets sold	1,113
Upfront consideration, inclusive of cash transferred of $2,898	277,898
Gain on sale of Cullinan Pearl	$ 276,785

Sale of Cullinan Pearl - Parent

Sale of Cullinan Pearl - Parenthetical (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Debt Securities, Available-for-Sale [Abstract]
Upfront consideration, cash transferred	$ 2,898

Reorganization and Reverse Stoc

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	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Stockholders' Equity (Members' Deficit)
Common Stock	$ 4	$ 5
Additional paid-in capital	654,685	585,320
Accumulated other comprehensive loss	(129)	(2,601)
Accumulated deficit	(200,857)	(47,695)
Total Cullinan Stockholders' Equity (Members' Deficit)	453,703	535,029
Non-controlling interests in subsidiaries	192	0
Total Stockholders' Equity (Members' Deficit)	$ 453,895	$ 535,029	$ 425,374

Financial Instruments - Schedul

Financial Instruments - Schedule of Short-term and Long-term Investment Marketable Securities by Security Type (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost	$ 368,762	$ 394,623
Gross Unrealized Gains	251	32
Gross Unrealized Losses	(380)	(2,633)
Estimated Fair Value	368,633	392,022
Short Term Investments
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost	368,762	313,187
Gross Unrealized Gains	251	14
Gross Unrealized Losses	(380)	(2,061)
Estimated Fair Value	368,633	311,140
Long Term Investments
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost		81,436
Gross Unrealized Gains		18
Gross Unrealized Losses		(572)
Estimated Fair Value		80,882
Asset-backed Securities | Short Term Investments
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost	61,114	16,625
Gross Unrealized Gains	3	0
Gross Unrealized Losses	(89)	(15)
Estimated Fair Value	61,028	16,610
U.S. Government Notes | Short Term Investments
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost	208,289	34,029
Gross Unrealized Gains	221	0
Gross Unrealized Losses	(16)	(290)
Estimated Fair Value	208,494	33,739
Corporate notes | Short Term Investments
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost	99,359	244,498
Gross Unrealized Gains	27	11
Gross Unrealized Losses	(275)	(1,743)
Estimated Fair Value	$ 99,111	242,766
Corporate notes | Long Term Investments
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost		81,436
Gross Unrealized Gains		18
Gross Unrealized Losses		(572)
Estimated Fair Value		80,882
Commercial Paper | Short Term Investments
Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Line Items]
Amortized Cost		18,035
Gross Unrealized Gains		3
Gross Unrealized Losses		(13)
Estimated Fair Value		$ 18,025

Financial Instruments - Summary

Financial Instruments - Summary of Fair Value of Financial Assets Measured on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	$ 368,633	$ 311,140
Long Term Investments Fair Value Disclosure		80,882
Total cash, cash equivalents and investments	368,633	392,022
Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	61,028	16,610
U.S. Government Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	208,494	33,739
Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	99,111	242,766
Long Term Investments Fair Value Disclosure		80,882
Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure		18,025
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	0	0
Long Term Investments Fair Value Disclosure		0
Total cash, cash equivalents and investments	0	0
Fair Value, Inputs, Level 1 [Member] | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	0	0
Fair Value, Inputs, Level 1 [Member] | U.S. Government Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	0
Long Term Investments Fair Value Disclosure		0
Fair Value, Inputs, Level 1 [Member] | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	0	0
Long Term Investments Fair Value Disclosure		0
Fair Value, Inputs, Level 1 [Member] | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure		0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	368,633	311,140
Long Term Investments Fair Value Disclosure		80,882
Total cash, cash equivalents and investments	368,633	392,022
Fair Value, Inputs, Level 2 [Member] | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	61,028	16,610
Fair Value, Inputs, Level 2 [Member] | U.S. Government Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	208,494	33,739
Fair Value, Inputs, Level 2 [Member] | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	99,111	242,766
Long Term Investments Fair Value Disclosure		80,882
Fair Value, Inputs, Level 2 [Member] | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure		18,025
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	0	0
Long Term Investments Fair Value Disclosure		0
Total cash, cash equivalents and investments	0	0
Fair Value, Inputs, Level 3 [Member] | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	0	0
Fair Value, Inputs, Level 3 [Member] | U.S. Government Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	0	0
Fair Value, Inputs, Level 3 [Member] | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure	$ 0	0
Long Term Investments Fair Value Disclosure		0
Fair Value, Inputs, Level 3 [Member] | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments Fair Value Disclosure		$ 0

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 1,341	$ 1,309
Less: accumulated depreciation	(352)	(135)
Total property and equipment, net	989	1,174
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	765	681
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 576	$ 628

Property and Equipment, Net (Ad

Property and Equipment, Net (Additional Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Depreciation Expense	$ 0.3	$ 0.1

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Accrued Expenses And Other Current Liabilities [Abstract]
Contracted research and development expenses	$ 8,434	$ 7,486
Due to Taiho under collaboration agreement, net	7,869	0
Employee compensation	6,987	4,516
Other current liabilities	914	1,955
Convertible note and accrued interest	0	178
Accrued Expenses And Other Current Liabilities	$ 24,204	$ 14,135

License and Collaboration Agr_2

License and Collaboration Agreements - Summary of Research and Development Costs Related to Collaboration and License Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs	$ 148,156	$ 91,948
Pearl - Taiho
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs	6,900	3,500
Collaboration and License Agreements | Adimab
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development costs	$ 500	$ 500

License and Collaboration Agr_3

License and Collaboration Agreements - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jun. 30, 2022	Dec. 31, 2019	Dec. 31, 2023	Dec. 31, 2022
License And Collaboration Agreements [Line Items]
Common stock, shares issued			42,900,083	45,796,449
Research and development			$ 148,156	$ 91,948
Accrued expenses and other current liabilities			2,404
Aggregate amount of milestone payment				100
Cash balance			2,898
Proceeds from issuance of common stock			38,388	0
Harbour License Agreement
License And Collaboration Agreements [Line Items]
Upfront license fee			25,000
Pearl - Taiho
License And Collaboration Agreements [Line Items]
Percentage of profits from potential sales	50%
Research and development expense related to share of costs			25,300	2,500
Research and development			6,900	3,500
Accrued expenses and other current liabilities			7,900	1,000
Harbour BioMed US Inc. | Harbour License Agreement
License And Collaboration Agreements [Line Items]
Upfront license fee			25,000
Harbour BioMed US Inc. | Achievement Based | Harbour License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment			148,000
Harbour BioMed US Inc. | Sales Based | Harbour License Agreement | Maximum
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment			415,000
Amber—Massachusetts Institute of Technology | Collaboration and License Agreements
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment			700
Amber—Massachusetts Institute of Technology | Collaboration and License Agreements | Maximum
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment			12,500
Amber—Massachusetts Institute of Technology | Clinical and Regulatory Events First Indication
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment			5,500
Amber—Massachusetts Institute of Technology | Clinical and Regulatory Events First Indication | Maximum
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment			7,000
Adimab | Collaboration and License Agreements
License And Collaboration Agreements [Line Items]
Research and development			500	500
Aggregate amount of milestone payment			500
Adimab | Collaboration and License Agreements | Maximum
License And Collaboration Agreements [Line Items]
Aggregate amount of milestone payment			15,800
Massachusetts Institute of Technology [Member] | Collaboration and License Agreements
License And Collaboration Agreements [Line Items]
Research and development			$ 800	$ 400
Massachusetts Institute of Technology [Member] | Clinical and Regulatory Events
License And Collaboration Agreements [Line Items]
Percentage of Milestone Payments		100%

Cullinan-MICA Asset Acquisition

Cullinan-MICA Asset Acquisition - Additional Information (Details) - shares	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Asset Acquisition [Line Items]
Common stock, shares authorized	150,000,000	150,000,000
Preferred stock shares authorized	10,000,000	10,000,000
Preferred stock shares outstanding	647,500	0
Common stock, shares outstanding	42,900,083	45,796,449

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Nov. 30, 2022	Oct. 31, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Jan. 31, 2023
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Common stock, shares authorized				150,000,000	150,000,000
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.
Increase in Number of Shares of Preferred Stock, Percentage				19.99%
Number of Shares of Common Stock, Percentage				9.99%
Preferred Stock, Par or Stated Value Per Share				$ 0.0001	$ 0.0001
Shares issued				647,500	0
Shares sold/issued				42,900,083	45,796,449
Losses attributed to noncontrolling interests				$ (1,939)	$ (2,019)
Proceeds from issuance of common stock				38,388	0
Equity-based compensation expense				$ 30,438	$ 27,957
Common Stock
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Shares issued during period				3,310,000
May Thirty One Two Thousend Thirty Three
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Offer and sell companys common stock				$ 125,000
May Thirty One Two Thousend Thirty Three | At The Market Equity Offering Program
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Treasury Stock, Common, Shares				85,600,000
Proceeds from Issuance Initial Public Offering				$ 38,400
Shares sold/issued				3,300,000
Biotechnology Value Fund, L.P exchange agreement [Member]
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Preferred Stock, Par or Stated Value Per Share						$ 0.0001
Number of common stock exchange						6,500,000
Convertible Preferred Stock, Shares Issued upon Conversion						600,000
Subsidiaries [Member]
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Losses attributed to noncontrolling interests	$ 33,300	$ 33,300	$ 1,200

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of development subsidiaries and their product candidates (Details) - Subsidiaries [Member]	Dec. 31, 2023
Cullinan Amber Corp [Member]
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Subsidiary, Ownership Percentage	94%
Cullinan Florentine Corp [Member]
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Subsidiary, Ownership Percentage	96%
Cullinan M I C A Corp [Member]
Common Stock And Noncontrolling Interest In Subsidiaries [Line Items]
Subsidiary, Ownership Percentage	95%

Leases - Additional Information

Leases - Additional Information (Details) $ in Thousands		12 Months Ended
	Aug. 31, 2022 ft²	Dec. 31, 2023 USD ($) ft²	Dec. 31, 2022 USD ($)
Lessee Lease Description [Line Items]
Operating lease commence date		Feb. 28, 2018
Operating lease expiration date		Jun. 30, 2024
Rent expense		$ 1,700	$ 1,100
Lease payments			300
Sublease payment		500	100
Impairment of long-lived assets		440	$ 0
Sublease Agreement
Lessee Lease Description [Line Items]
Impairment of long-lived assets		$ 400
M A
Lessee Lease Description [Line Items]
Operating lease rentable area | ft²	14,000	8,000

Leases - Supplemental cash flow

Leases - Supplemental cash flow information - (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Operating cash flows from operating leases(1)	[1]	$ 1,881	$ 260
ROU asset obtained in exchange for an operating lease liability		$ 0	$ 4,931
[1] Operating cash flows from operating leases for 2022 includes cash inflow of $ 0.3 million reimbursed by the lessor for improvements made to the newly leased office space pursuant to the terms of the Suite 1350 lease.

Leases - Summarizes lease term

Leases - Summarizes lease term and discount rate - (Details)	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Weighted-average remaining lease term (in years)	2 years 4 months 24 days	3 years 2 months 12 days
Weighted-average discount rate	10.90%	10.80%

Leases - Future minimum lease p

Leases - Future minimum lease payments - (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Leases [Abstract]
2024	$ 1,738
2025	1,461
2026	872
Total future minimum lease payments	4,071
Less: imputed interest	(481)
Total lease liabilities at present value	$ 3,590

Equity-Based Compensation - Sch

Equity-Based Compensation - Schedule of Equity-based Compensation Expense Categories In Consolidated Statements of Operations and Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total equity-based compensation	$ 30,438	$ 27,957
Research and Development
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total equity-based compensation	12,168	11,018
General and Administrative
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total equity-based compensation	$ 18,270	$ 16,939

Equity-Based Compensation - Add

Equity-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2022	Dec. 31, 2023	Dec. 31, 2022		Jan. 01, 2024
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted term		7 years 11 months 8 days
Options granted, weighted average grant date fair value		$ 7.7	$ 9.02
Unrecognized compensation costs		$ 50,900	$ 66,000
Unrecognized compensation costs, recognition period		2 years 4 months 24 days	2 years 10 months 24 days
Options exercised, intrinsic value		$ 300	$ 13,100
Common stock, par value		$ 0.0001	$ 0.0001
Common stock, shares issued		42,900,083	45,796,449
Equity-based compensation expense		$ 30,438	$ 27,957
Proceeds from issuance of common stock		$ 38,388	$ 0
Common stock issuable upon exercise of stock options, weighted average exercise price		$ 4.3	$ 4.31
Equity-based compensation expense		$ 30,438	$ 27,957
Research and Development Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Equity-based compensation expense		12,168	11,018
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Equity-based compensation expense		$ 18,270	$ 16,939
Cullinan's CLN-619 [Member] | HLBV
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares price per share		$ 0.22
Share-based payment award, accelerated vesting, number	300,000
Equity-based compensation expense	$ 600
Stock options held by noncontrolling interests		200,000
RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation costs, recognition period		2 years 6 months	2 years 7 months 6 days
Non-options unrecognized compensation cost		$ 6,800	$ 4,300
Fair value of non-options shares vested		$ 1,700	$ 600
Shares price per share			$ 12.98
Weighted average fair value of options granted		$ 11.29	$ 13.28	[1]
Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation costs, recognition period			1 year
Non-options unrecognized compensation cost		$ 0	$ 100
Fair value of non-options shares vested		$ 100	$ 600
2021 Stock Option and Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted term		10 years
Award vesting period		4 years
Common stock, shares reserved for future issuance		1,300,000
Number of shares available for grant		2,600,000
Increase in number of shares reserved and available for issuance percentage		5%
2021 Stock Option and Incentive Plan | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in number of shares reserved and available for issuance					2,100,000
ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in number of shares reserved and available for issuance percentage		1%
Increase in number of shares reserved and available for issuance		800,000
ESPP | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in number of shares reserved and available for issuance					0.4
ESPP | Maximum | Cullinan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued		100,000	100,000
[1] The number granted represents the number of shares issuable upon vesting of service-based and market-based RSUs, assuming the Company achieves its corporate stock price metrics at the target achievement level.

Equity-Based Compensation - S_2

Equity-Based Compensation - Schedule of Assumptions Used to Estimate Fair Value of Options (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]
Risk-free interest rate	4%	2.70%
Expected term (in years)	6 years	6 years
Expected volatility	78.70%	79.80%
Expected dividend yield	0%	0%

Equity-Based Compensation - S_3

Equity-Based Compensation - Schedule of Assumptions Used to Estimate Fair Value of RSUs (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Volatility	78.70%	79.80%
Expected term (in years)	6 years	6 years
Risk-free rate	4%	2.70%
Expected dividend yield	0%	0%
RSUs
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock price		$ 12.98
Volatility		82.50%
Expected term (in years)		2 years 8 months 12 days
Risk-free rate		2.90%
Expected dividend yield		0%

Equity-Based Compensation - Sum

Equity-Based Compensation - Summary of Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]
Number of Options, Beginning Balance	9,334,000	9,259,000
Number of Options, Granted	2,091,000	2,888,000
Number of Options, Exercised	(58,000)	(1,523,000)
Number of Options, Forfeited	(828,000)	(1,290,000)
Number of Options, Ending Balance	10,539,000	9,334,000
Number of Options, Exercisable	5,535,000
Weighted Average Exercise Price, Beginning Balance	$ 17.04	$ 15.77
Weighted Average Exercise Price, Granted	10.96	12.96
Weighted Average Exercise Price, Exercised	4.3	4.31
Weighted Average Exercise Price, Forfeited	22.63	13.8
Weighted Average Exercise Price, Ending Balance	15.46	$ 17.04
Weighted Average Exercise Price, Exercisable	$ 15.37
Weighted Average Remaining Contractual Term (Years), Outstanding	7 years 11 months 8 days
Weighted Average Remaining Contractual Term (Years), Exercisable	7 years 5 months 26 days
Aggregate Intrinsic Value, Outstanding	$ 10,753
Aggregate Intrinsic Value, Exercisable	$ 10,170

Equity-Based Compensation - S_4

Equity-Based Compensation - Summary of Activity Related to RSUs (Details) - RSUs - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of Shares, Outstanding Unvested, Beginning Balance	369,000	0
Number of Shares, Granted	638,000	502,000	[1]
Number of Shares, Vested	(178,000)	(46,000)
Number of Shares, Forfeited	(25,000)	(87,000)
Number of Shares, Outstanding Unvested, Ending Balance	804,000	369,000
Weighted-Average Grant Date Fair Value, Outstanding Unvested, Beginning Balance	$ 13.16	$ 0
Weighted-Average Grant Date Fair Value, Granted	11.29	13.28	[1]
Weighted-Average Grant Date Fair Value, Vested	11.9	13.6
Weighted-Average Grant Date Fair Value, Forfeited	12.26	13.6
Weighted-Average Grant Date Fair Value, Outstanding Unvested, Ending Balance	$ 11.98	$ 13.16
[1] The number granted represents the number of shares issuable upon vesting of service-based and market-based RSUs, assuming the Company achieves its corporate stock price metrics at the target achievement level.

Equity-Based Compensation - S_5

Equity-Based Compensation - Summary of Activity Related to RSAs (Details) - Restricted Stock Awards - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of Shares, Outstanding Unvested, Beginning Balance	15	89
Number of Shares, Vested	(14)	(46)
Number of Shares, Forfeited	(1)	(28)
Number of Shares, Outstanding Unvested, Ending Balance	0	15
Weighted-Average Grant Date Fair Value, Outstanding Unvested, Beginning Balance	$ 3.87	$ 3.87
Weighted-Average Grant Date Fair Value, Vested	3.87	3.87
Weighted-Average Grant Date Fair Value, Forfeited	3.87	3.87
Weighted-Average Grant Date Fair Value, Outstanding Unvested, Ending Balance	$ 0	$ 3.87

Equity-Based Compensation - S_6

Equity-Based Compensation - Summary of Restricted Common Unit Activities (Details) - RSUs - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Vested	(178,000)	(46,000)
Number of Shares, Outstanding Unvested, Ending Balance	804,000	369,000

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Royalties under obligation	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Line Items]
Current income tax expense (benefit)	$ 14,100	$ 42,100
Deferred Income Tax Expense (Benefit)	5,400
Other Assets, Current	3,900
Operating loss carryforwards	$ 54,300	42,600
Operating loss carryforwards, limitations on use	Under Sections 382 and 383 of the Internal Revenue Code of 1986, as amended, as well as similar state provisions, NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company.
Uncertain tax positions	$ 0	0
Accrued interest or penalties	0	0
Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforward	$ 1,500
Federal
Income Tax Disclosure [Line Items]
Operating loss carryforwards expiration year	2037
Federal | Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforward	$ 1,600	900
Tax credit carryforward expiration year	2036
Federal | Research and Development Tax Credit Carryforward Indefinitely
Income Tax Disclosure [Line Items]
Operating loss carryforwards expiration year	2036
Federal | Prior to 2018 Tax Year
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 1,400
Federal | NOL Current and Prior to 2018 Tax Year
Income Tax Disclosure [Line Items]
Operating loss carryforwards	52,900
Federal | NOL Current and Prior to 2018 Tax Year | Research and Development Tax Credit Carryforward Indefinitely
Income Tax Disclosure [Line Items]
Tax credit carryforward	300
Tax Credit Carry Forward Expired	1,700
State
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 56,800	27,100
Operating loss carryforwards expiration year	2031
State | NOL Current and Prior to 2018 Tax Year | Research and Development Tax Credit Carryforward Indefinitely
Income Tax Disclosure [Line Items]
Tax credit carryforward	$ 2,000	$ 300

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of Statutory Income Tax Rate To Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	21%	21%
State taxes, net of federal benefit	6.70%	4.71%
Research and development credits	6.28%	(0.25%)
Equity-based compensation	(1.49%)	0.77%
Valuation allowance	(24.41%)	1.13%
IPR&D	(6.28%)	0.25%
Other	0.27%	0.45%
Effective tax rate	8.35%	27.81%

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Income Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Capitalized research and development	$ 41,901	$ 18,724
Equity-based compensation	16,960	12,087
Net operating loss	14,443	10,531
Licenses	7,189	461
Research and development credit	3,155	1,110
Accrued expenses	1,836	1,977
Basis difference on gain on sale of Cullinan Pearl	1,700	1,805
Lease liability	985	1,345
Capitalized organizational and start-up expenses	116	127
Gross deferred tax assets	88,285	48,167
Valuation allowance	(87,371)	(46,766)	$ (44,552)
Net deferred tax asset	914	1,401
Deferred tax liability
ROU asset	697	1,108
Depreciation and amortization	217	293
Net deferred tax asset	$ 0	$ 0

Income Taxes - Summary of Valua

Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Valuation Allowance [Abstract]
Valuation allowance at beginning of year	$ 46,766	$ 44,552
Increases recorded to income tax provision	41,307	1,712
Increases recorded to equity	(702)	502
Valuation allowance at end of year	$ 87,371	$ 46,766

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Indemnification agreements
Lessee Lease Description [Line Items]
Accrued liabilities	$ 0	$ 0

Net Income (Loss) per Share A_3

Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan - Summary of Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Numerator:
Net income (loss) attributable to common stockholders of Cullinan	$ (153,162)	$ 111,214
Denominator
Weighted-average common stock outstanding - basic	41,550	45,164
Dilutive effect of common stock issuable from assumed exercise of equity awards	0	1,476
Weighted-average common stock outstanding - diluted	41,550	46,640
Basic	$ (3.69)	$ 2.46
Diluted	$ (3.69)	$ 2.38

Net Income (Loss) per Share A_4

Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan - Summary of Equity Instruments Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	15,712	6,874
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	9,418	6,842
Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	6,138	0
RSAs and RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	147	25
ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	9	7

Subsequent Events (Additional I

Subsequent Events (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Subsequent Event [Line Items]
Preferred stock, par value	$ 0.0001	$ 0.0001
Common stock, shares outstanding	42,900,083	45,796,449
Preferred stock shares outstanding	647,500	0
Milestone payments received		$ 0.1
Harbour License Agreement | Harbour BioMed US Inc. | Maximum | Achievement Based
Subsequent Event [Line Items]
Milestone payments received	$ 148
Harbour License Agreement | Harbour BioMed US Inc. | Maximum | Sales Based
Subsequent Event [Line Items]
Milestone payments received	$ 415