SEC Form POSASR filed by Arcellx Inc.

As filed with the Securities and Exchange Commission on April 28, 2026

Registration No. 333-271731

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

POST- EFFECTIVE AMENDMENT NO. 1

TO

FORM S-3

REGISTRATION STATEMENT NO. 333-271731

UNDER

THE SECURITIES ACT OF 1933

Arcellx, Inc.

(Exact name of registrant as specified in its charter)

800 Bridge Parkway

Redwood City, CA 94065

(240) 327-0630

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Andrew D. Dickinson
President and Treasurer

Arcellx, Inc.

333 Lakeside Drive
Foster City, CA 94404

(240) 327-0630

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy to:

Emily Oldshue

Nicholas Roper

Ropes & Gray LLP

Prudential Tower, 800 Boylston Street

Boston, Massachusetts 02199

(617) 951-7000

(Approximate date of commencement of proposed sale to the public): Not applicable.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box: ¨

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box:  ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.  x

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ¨

DEREGISTRATION OF SECURITIES

This Post-Effective Amendment (this “Post-Effective Amendment”) is being filed by Arcellx, Inc. (the “Registrant”) to deregister any and all securities, registered but unsold or otherwise unissued as of the date hereof under the following Registration Statement on Form S-3 (the “Registration Statement”) previously filed by the Registrant with the U.S. Securities and Exchange Commission (the “SEC”):

On April 28, 2026, pursuant to an Agreement and Plan of Merger (the “Merger Agreement”), dated as of February 22, 2026, by and among the Registrant, Gilead Sciences, Inc. (“Parent”) and Ravens Sub, Inc. (“Merger Sub”), Merger Sub merged with and into the Registrant (the “Merger”), with the Registrant continuing as the surviving corporation and becoming a wholly owned subsidiary of Parent. As a result of the transactions contemplated by the Merger Agreement, the Registrant has terminated any and all offerings and sales of securities pursuant to the Registration Statement. In accordance with the undertakings made by the Registrant in the Registration Statement to remove from registration, by means of a post-effective amendment, any of the securities of the Registrant registered under the Registration Statement which remain unsold at the termination of the offering, the Registrant hereby terminates the effectiveness of the Registration Statement and removes from registration all of the securities that remain unsold under the Registration Statement as of the date hereof, if any. The Registration Statement is hereby amended, as appropriate, to reflect the deregistration of such securities. After giving effect to this Post-Effective Amendment, there will be no remaining securities registered by the Registrant pursuant to the Registration Statement.

The foregoing description of the Merger, the Merger Agreement and the transactions contemplated thereby does not purport to be complete and is subject to, and qualified in its entirety by, the Merger Agreement, which is attached as Exhibit 2.1 to the Registrant’s Current Report on Form 8-K filed with the SEC on February 23, 2026.

SIGNATURE

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this Post-Effective Amendment to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Redwood City, State of California, on April 28, 2026.

No other person is required to sign this Post-Effective Amendment in reliance upon Rule 478 under the Securities Act of 1933, as amended.