Dyne CEO auto-sells 3,311 shares for tax withholding

Dyne Therapeutics CEO John Cox reported an automatic tax-related sale of 3,311 shares of common stock.

Rhea-AI Filing Summary

Dyne Therapeutics CEO John Cox reported an automatic tax-related sale of 3,311 shares of common stock. The shares were sold on May 13, 2026 at a weighted average price of $18.36 per share to satisfy tax withholding obligations tied to the vesting of restricted stock units granted on February 12, 2026.

The sale was executed under a restricted stock unit agreement described as a binding contract consistent with the Rule 10b5-1 affirmative defense, meaning it was not a discretionary trade. After this transaction, Cox directly holds 370,834 shares, including 268,645 unvested RSUs, and also reports indirect holdings through trusts for the benefit of a child.

Insider Trade Summary 10b5-1

Net Seller: 3,311 shares

Net Sell 5 txns

Insider Cox John

Role CEO & President

Sold 3,311 shs ($61K)

Type	Security	Shares	Price	Value
Sale	Common Stock	3,311	$18.36	$61K
holding	Common Stock	--	--	--
holding	Common Stock	--	--	--
holding	Common Stock	--	--	--
holding	Common Stock	--	--	--

Holdings After Transaction: Common Stock — 370,834 shares (Direct); Common Stock — 18,000 shares (Indirect, By Trust #1); Common Stock — 18,000 shares (Indirect, By Trust #2); Common Stock — 18,000 shares (Indirect, By Trust #3); Common Stock — 18,000 shares (Indirect, By Trust #4)

Footnotes (4)

1. F1. Represents shares automatically sold by the Reporting Person to satisfy tax withholding obligations in connection with the vesting of restricted stock units granted to the Reporting Person on February 12, 2026. The automatic sale of the Reporting Person's shares is provided for in a restricted stock unit agreement constituting a "binding contract" consistent with the affirmative defense to liability under Rule 10b5-1 and the sale does not represent a discretionary trade by the Reporting Person.
2. F2. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices within the range of $18.20 to $18.41, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote of this Form 4.
3. F3. Includes 268,645 unvested RSUs.
4. F4. These shares are held in a trust for the benefit of a child of the Reporting Person.

Key Figures

Shares sold: 3,311 shares Average sale price: $18.36 per share Post-transaction direct holdings: 370,834 shares +2 more

5 metrics

Shares sold 3,311 shares Automatic sale on May 13, 2026 for tax withholding

Average sale price $18.36 per share Weighted average price for the 3,311 shares sold

Post-transaction direct holdings 370,834 shares Common stock directly held after the sale

Unvested RSUs 268,645 RSUs Included within Cox’s direct holdings after the sale

Trust-held shares 18,000 shares Common stock held indirectly in a trust for a child

Key Terms

restricted stock units, Rule 10b5-1, weighted average price, unvested RSUs, +1 more

5 terms

restricted stock units financial

"vesting of restricted stock units granted to the Reporting Person on February 12, 2026"

Restricted stock units are a type of company reward where employees are promised shares of stock, but they only fully own these shares after meeting certain conditions, like staying with the company for a set time. They matter because they can become valuable assets and are often used to motivate employees to help the company succeed.

Rule 10b5-1 regulatory

"binding contract consistent with the affirmative defense to liability under Rule 10b5-1"

Rule 10b5-1 is a regulation that allows company insiders to buy or sell their shares at predetermined times, even if they have access to non-public information. It acts like setting a schedule in advance for transactions, helping prevent accusations of unfair trading. This rule provides a way for insiders to plan trades transparently, giving investors confidence that these transactions are not based on hidden information.

weighted average price financial

"The price reported in Column 4 is a weighted average price."

Weighted average price is the average price of a security where each trade or component is counted according to its size, so bigger trades pull the average more than smaller ones. Think of it like calculating the average cost of a grocery haul where items you bought more of have greater influence on the final per-item cost. Investors use it to understand the true average price paid or received, judge execution quality, and compare trading performance against market movement.

unvested RSUs financial

"Includes 268,645 unvested RSUs."

trust financial

"These shares are held in a trust for the benefit of a child of the Reporting Person."

A trust is a legal setup in which one party (the trustee) holds and manages assets—like cash, stocks or property—on behalf of other people (beneficiaries) according to instructions from the person who created it (the grantor). Think of it as a locked box with a keyholder who must follow written rules; for investors it matters because trusts influence who controls and benefits from assets, affect taxes and succession, and can change how quickly or transparently shares are bought, sold or voted.

FAQ

What insider transaction did Dyne Therapeutics (DYN) report for CEO John Cox?

Dyne Therapeutics reported that CEO John Cox had 3,311 shares of common stock sold. The sale was automatic to cover tax withholding obligations triggered by the vesting of previously granted restricted stock units, not a discretionary open-market trade by Cox.

At what price were John Cox’s Dyne Therapeutics (DYN) shares sold?

The 3,311 Dyne Therapeutics shares tied to John Cox’s tax withholding were sold at a weighted average price of $18.36. Footnotes state individual trades occurred between $18.20 and $18.41 per share, reflecting multiple executions within that range on the transaction date.

Why were Dyne Therapeutics (DYN) shares sold from John Cox’s holdings?

The shares were sold solely to satisfy tax withholding obligations from the vesting of restricted stock units granted on February 12, 2026. The filing explains this automatic sale was pre-arranged under a binding RSU agreement consistent with Rule 10b5-1 protections.

How many Dyne Therapeutics (DYN) shares does John Cox hold after the reported sale?

After the tax-related sale, John Cox directly holds 370,834 Dyne Therapeutics common shares. This total includes 268,645 unvested restricted stock units, showing that the reported 3,311-share sale is small relative to his remaining direct equity position in the company.

Were John Cox’s Dyne Therapeutics (DYN) share sales discretionary trades?

The filing states the 3,311-share sale was not discretionary. It was automatically executed under a restricted stock unit agreement designed as a binding contract consistent with Rule 10b5-1, specifically to cover tax withholding obligations on RSU vesting for John Cox.

Does John Cox have indirect Dyne Therapeutics (DYN) holdings through trusts?

Yes. The Form 4 notes that some Dyne Therapeutics shares are held indirectly in trusts for the benefit of a child of John Cox. These trust holdings are reported as indirect ownership separate from his 370,834 directly held shares following the tax-related sale.

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Accepted by SEC EDGAR 05/14/2026 - 04:02 PM Learn about SEC filing dates

SEC Form 4

FORM 4

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
or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL

OMB Number:			3235-0287
Estimated average burden
hours per response:			0.5

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

X

Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

1. Name and Address of Reporting Person^*

Cox John

(Last) (First) (Middle)

C/O DYNE THERAPEUTICS, INC.

1560 TRAPELO ROAD

(Street)

WALTHAM MASSACHUSETTS 02451

(City) (State) (Zip)

UNITED STATES

(Country)

2. Issuer Name and Ticker or Trading Symbol
Dyne Therapeutics, Inc. [ DYN ]

5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

X	Director		10% Owner
X	Officer (give title below)		Other (specify below)
	CEO & President

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
05/13/2026

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

4. If Amendment, Date of Original Filed (Month/Day/Year)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	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	05/13/2026		S(1)		3,311	D	$18.36(2)	370,834(3)	D
Common Stock								18,000	I	By Trust #1(4)
Common Stock								18,000	I	By Trust #2(4)
Common Stock								18,000	I	By Trust #3(4)
Common Stock								18,000	I	By Trust #4(4)

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 (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	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 (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (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: 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

Explanation of Responses:

1. Represents shares automatically sold by the Reporting Person to satisfy tax withholding obligations in connection with the vesting of restricted stock units granted to the Reporting Person on February 12, 2026. The automatic sale of the Reporting Person's shares is provided for in a restricted stock unit agreement constituting a "binding contract" consistent with the affirmative defense to liability under Rule 10b5-1 and the sale does not represent a discretionary trade by the Reporting Person.

2. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices within the range of $18.20 to $18.41, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote of this Form 4.

3. Includes 268,645 unvested RSUs.

4. These shares are held in a trust for the benefit of a child of the Reporting Person.

	/s/ Ron Caponigro, Attorney-in-Fact	05/14/2026
	** Signature of Reporting Person	Date
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).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.
* Form 4: SEC 1474 (03-26)