Changes in Beneficial Ownership · 4

TRex Bio, Inc.

TRXBNASDAQEQUITYCurrent

Changes in Beneficial Ownership

Filed Oct 13, 2026Accepted Oct 13, 2026, 6:31 PM EDTFiling CIK 2139097Accession 0001193125-26-420734

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Structured filing — 4

ownership.xml

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Filing details

Report period: 2026-10-13
Issuer: TRex Bio, Inc.
Issuer CIK: 0002139097
Trading symbol: TRXB

Reporting owner 1

Name: MARCUS JOEL S
Reporting owner CIK: 0001216955
Relationship: Director
Address: C/O TREX BIO, INC., 269 EAST GRAND AVENUE, SUITE 300, SOUTH SAN FRANCISCO, CA, 94080

Non-derivative transactions

Non-derivative transactions · 1 reported row
Security	Transaction date	Code	Amount	A / D	Price (USD)	Owned after	Ownership
Common Stock	2026-10-13	C · Form 4	41,871	A	^[F1]	41,871	D

Table key

C · Form 4: Derivative conversion
A: Acquired
D: Direct

Derivative transactions

Derivative transactions · 1 reported row
Security	Conversion / exercise price (USD)	Transaction date	Code	Amount	A / D	Price (USD)	Exercisable	Expires	Underlying security	Underlying shares	Owned after	Ownership
Series B Convertible Preferred Stock	^[F1]	2026-10-13	C · Form 4	161,828	D	0	^[F1]	^[F1]	Common Stock	41,871	0	D

Table key

C · Form 4: Derivative conversion
D: Direct

Footnotes

F1: On October 13, 2026, the shares of Series B Convertible Preferred Stock automatically converted into shares of Common Stock on a 3.8649-to-1 basis without payment of further consideration immediately prior to the closing of the Issuer's initial public offering. The shares have no expiration date.↩ 1 ↩ 2 ↩ 3 ↩ 4

Signature 1

Signed: /s/ Brandon Hants, as attorney-in-fact
Date: 2026-10-13

Company context

We are a clinical-stage biopharmaceutical company focused on tissue Treg biology for the discovery and development of immunoregulatory medicines designed to restore immune balance and promote tissue repair to address unmet needs of patients with autoimmune and inflammatory diseases. TRexBio was built on a pivotal scientific insight: that regulatory T cells (“Tregs”) in human tissue are not merely passive suppressors of inflammation, but active orchestrators of tissue immune homeostasis and repair. Leveraging our Deep Biology drug discovery and design platform, we integrate tissue-level insights into human biology and Treg function to identify and characterize novel targets and approaches for therapeutic intervention. Our tissue-first conviction and unique insights in Treg biology are the foundation of our pipeline of therapeutic candidates aimed at restoring immune balance and promoting tissue repair for more durable disease control.

Current securities

TRXBNASDAQ · EQUITY · Current

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