Sec Form 4 Filing - VIKING GLOBAL INVESTORS LP @ Adaptive Biotechnologies Corp - 2020-12-14

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person VIKING GLOBAL INVESTORS LP 2. Issuer Name and Ticker or Trading Symbol Adaptive Biotechnologies Corp [ ADPT] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 55 RAILROAD AVENUE 3. Date of Earliest Transaction (MM/DD/YY) 12/14/2020 (Street) GREENWICH, CT06830 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 12/14/2020 S 1,000,000 ( 1 ) D $ 55.65 29,993,708 I See explanation of responses ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other VIKING GLOBAL INVESTORS LP 55 RAILROAD AVENUE GREENWICH, CT06830 X VIKING GLOBAL PERFORMANCE LLC 55 RAILROAD AVENUE GREENWICH, CT06830 X Viking Global Equities II LP 55 RAILROAD AVENUE GREENWICH, CT06830 X Viking Global Equities Master Ltd. 55 RAILROAD AVENUE GREENWICH, CT06830 X Viking Long Fund GP LLC 55 RAILROAD AVENUE GREENWICH, CT06830 X Viking Long Fund Master Ltd. 55 RAILROAD AVENUE GREENWICH, CT06830 X HALVORSEN OLE ANDREAS 55 RAILROAD AVENUE GREENWICH, CT06830 X Ott David C. 55 RAILROAD AVENUE GREENWICH, CT06830 X Shabet Rose Sharon 55 RAILROAD AVENUE GREENWICH, CT06830 X Signatures /s/ O. Andreas Halvorsen (8) (9) 12/16/2020 Signature of Reporting Person Date /s/ David C. Ott (8) (9) 12/16/2020 Signature of Reporting Person Date /s/ Rose S. Shabet (8) (9) 12/16/2020 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On December 14, 2020, the Viking Funds (defined below) sold an aggregate of 1,000,000 shares of common stock of the Issuer ("Common Stock") at a price of $55.65 per share.VGE II sold 14,671 shares, VGEM sold 718,897 shares and VLFM sold 266,432 shares. ( 2 )O. Andreas Halvorsen, David C. Ott and Rose S. Shabet are Executive Committee members of certain management entities, including Viking Global Partners LLC, the general partner of Viking Global Investors LP ("VGI"), Viking Global Performance LLC ("VGP") and Viking Long Fund GP LLC ("VLFGP"). VGI provides managerial services to various investment funds, including Viking Global Equities II LP ("VGE II") and Viking Global Equities Master Ltd. ("VGEM") and Viking Long Fund Master Ltd. ("VLFM" and together with VLFM, VGEM and VGE II, the "Viking Funds"). ( 3 )VGP is the general partner of VGE II. VLFGP serves as the investment manager of VLFM. Each of the Viking Funds, VGI, VGP, VLFGP, Mr. Halvorsen, Mr. Ott and Ms. Shabet (collectively, the "Reporting Persons") may be deemed to beneficially own the shares reported on this form. ( 4 )VGI provides managerial services to (i) VGE II, which directly holds 454,717 shares of Common Stock; (ii) VGEM, which directly holds 22,281,040 shares of Common Stock; and (iii) VLFM, which directly holds 8,257,951 shares of Common Stock. Because of the relationship between VGI and the Viking Funds, VGI may be deemed to beneficially own the shares of Common Stock directly held by the Viking Funds. ( 5 )VGP is the general partner of VGE II and the investment manager of VGEM. Because of the relationships between VGP and VGE II and VGEM, respectively, VGP may be deemed to beneficially own the shares of Common Stock directly held by VGE II and VGEM. ( 6 )VLFGP serves as the investment manager of VLFM. Because of the relationship between VLFGP and VLFM, VLFGP may be deemed to beneficially own the shares of Common Stock directly held by VLFM. ( 7 )The Reporting Persons disclaim beneficial ownership of these securities except to the extent of their pecuniary interest therein, and the inclusion of these securities in this report shall not be deemed an admission of beneficial ownership of the reported securities for purposes of Section 16 or for any other purpose.

Remarks: 8. The Reporting Persons are jointly filing this Form 4 pursuant to Rule 16a-3(j) under the Exchange Act. 9. Each of O. Andreas Halvorsen, David C. Ott and Rose S. Shabet is signing individually and as an a Executive Committee Member of VIKING GLOBAL PARTNERS LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING GLOBAL PERFORMANCE LLC, on behalf of itself and VIKING GLOBAL EQUITIES II LP and VIKING GLOBAL EQUITIES MASTER LTD., and as an Executive Committee Member of VIKING LONG FUND GP LLC, on behalf of itself and VIKING LONG FUND MASTER LTD.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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