Dyne Therapeutics Announces FDA Breakthrough Therapy Designation for DYNE-101 and Updated Plan for Accelerated Approval in DM1 Following Type C Meeting - Based on Type C meeting and new data, Dyne submitted revised ACHIE

Dyne Therapeutics Announces FDA Breakthrough Therapy Designation for

DYNE-101 and Updated Plan for Accelerated Approval in DM1 Following Type C Meeting

Based on Type C meeting and new data, Dyne submitted revised ACHIEVE trial protocol to FDA elevating vHOT to primary endpoint for U.S. Accelerated Approval -

- New positive clinical data from Phase 1/2 ACHIEVE trial support vHOT as early indicator of clinical benefit with DYNE-101 in DM1 -

- Ongoing Registrational Expansion Cohort in ACHIEVE trial to enroll 60

participants and include sites in U.S. -

- Company to host an investor and analyst conference call today, June 17, at 8:00 a.m.

WALTHAM, Mass., June 17, 2025 Dyne Therapeutics, Inc. (Nasdaq: DYN), a clinical-stage company focused on delivering

functional improvement for people living with genetically driven neuromuscular diseases, today announced that the U.S. Food and Drug Administration (FDA) has granted Breakthrough Therapy Designation to

DYNE-101 for the treatment of myotonic dystrophy type 1 (DM1). The company also announced an updated plan for obtaining U.S. Accelerated Approval for DYNE-101 in DM1

following a Type C meeting with the FDA and analysis of new long-term functional data.

After our Type C meeting, we were granted Breakthrough

Therapy Designation for DYNE-101 in DM1. We appreciate the FDA s active engagement and guidance as we advance this promising program through the Accelerated Approval pathway in the U.S., said John

Cox, president and chief executive officer of Dyne. Based on feedback from the FDA, along with our 6-month and new 12-month efficacy data, we have submitted a

revised protocol for the ongoing Registrational Expansion Cohort of the ACHIEVE trial with vHOT as the primary endpoint for potential Accelerated Approval.

Accelerated Approval Pathway for DYNE-101 in DM1

Accelerated Approval Milestones for DYNE-101 in DM1

New Long-term Data from Multiple Ascending Dose (MAD) Portion of ACHIEVE Trial

Updated Cash Runway Guidance

The company expects that its cash, cash equivalents and marketable securities as of March 31, 2025 will be sufficient to fund its operations into the

fourth quarter of 2026. As previously reported, cash, cash equivalents and marketable securities were $677.5 million as of March 31, 2025.

Investor Conference Call

Dyne will host a conference

call and webcast to discuss these updates today, June 17, 2025, at 8:00 a.m. ET and a replay will be accessible for 90 days following the presentation. An accompanying slide presentation for the event and an updated corporate presentation will

also be available. To access these presentations and register for the webcast and replay, please visit the Investors & Media section of Dyne s website at https://investors.dyne-tx.com/news-and-events/events-and-presentations.

About U.S. FDA Accelerated Approval

Accelerated Approval allows the FDA to approve drugs for serious conditions with an unmet medical need based on a surrogate or intermediate clinical endpoint,

which is reasonably likely to predict clinical benefit.

About Breakthrough Therapy Designation

The FDA grants Breakthrough Therapy Designation to expedite the development and review of drugs that are intended to treat a serious condition with preliminary

clinical evidence indicating that the drug may demonstrate substantial improvement over available therapy on one or more clinically significant endpoints. This designation offers benefits to DYNE-101 in the

DYNE-101 is an investigational therapeutic being evaluated in the Phase 1/2 global ACHIEVE clinical trial for people

living with DM1. DYNE-101 consists of an antisense oligonucleotide (ASO) conjugated to a fragment antibody (Fab) that binds to the transferrin receptor 1 (TfR1) to enable delivery to muscle and the central

nervous system. It is designed to promote functional improvement in individuals living with DM1 by reducing toxic nuclear DMPK RNA and correcting the spliceopathy underlying the disease. DYNE-101 has

been granted Orphan drug, Fast Track and Breakthrough Therapy designations by the U.S. Food and Drug Administration and Orphan drug designation by the European Medicines Agency for the treatment of DM1.

About Myotonic Dystrophy Type 1 (DM1)

progressive, genetic disease that affects skeletal, cardiac and smooth muscle in addition to the central nervous system (CNS). It is a monogenic, autosomal dominant disease caused by an abnormal trinucleotide expansion in a region of the DMPK

gene. This expansion of CTG repeats causes toxic RNA to cluster in the nucleus, forming nuclear foci and altering the splicing of multiple proteins essential for normal cellular function. This altered splicing, or spliceopathy, results in a wide

range of symptoms. People living with DM1 typically experience myotonia and progressive weakness of major muscle groups, which can affect mobility, breathing, heart function, speech, digestion and vision. Cognitive dysfunction may manifest as

fatigue, excessive daytime sleepiness, an apathic temperament and brain fog. DM1 is estimated to affect more than 40,000 people in the United States and over 74,000 people in Europe, but there are currently no approved disease-modifying

About Dyne Therapeutics

Dyne Therapeutics is focused on delivering functional improvement for people living with genetically driven neuromuscular diseases. We are developing

therapeutics that target muscle and the central nervous system (CNS) to address the root cause of disease. The company is advancing clinical programs for myotonic dystrophy type 1 (DM1) and Duchenne muscular dystrophy (DMD), and preclinical programs

for facioscapulohumeral muscular dystrophy (FSHD) and Pompe disease. At Dyne, we are on a mission to deliver functional improvement for individuals, families and communities. Learn more

Forward-Looking Statements

This press release contains

forward-looking statements that involve substantial risks and uncertainties. All statements, other than statements of historical facts, contained in this press release, including statements regarding Dyne s strategy, future operations,

prospects and plans, objectives of management, the potential of the FORCE platform and of DYNE-101, the anticipated timelines for reporting additional data from the ACHIEVE clinical trial and for the

initiation of the planned phase 3 clinical trial in patients with DM1, initiating and enrolling both clinical trials, initiating additional clinical trials, and submitting applications for marketing approval, the availability of expedited approval

pathways for DYNE-101, expectations regarding the outcome of interactions with regulatory authorities, and the sufficiency of Dyne s cash resources for the period anticipated, constitute forward-looking

statements within the meaning of The Private Securities Litigation Reform Act of 1995. The words anticipate, believe, continue, could, estimate, expect, intend,

may, might, objective, ongoing, plan, predict, project, potential, should, or would, or the negative of these terms, or other

comparable terminology are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. Dyne may not actually achieve the plans, intentions or expectations disclosed in these

forward-looking statements, and you should not place undue reliance on these forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in these forward-looking statements as a

result of various important factors, including: uncertainties inherent in the identification and development of product candidates, including the initiation and completion of preclinical studies and clinical trials; uncertainties as to the

availability and timing of results from preclinical studies and clinical trials; the timing of and Dyne s ability to enroll patients in clinical trials; whether results from preclinical studies and data from clinical trials will be predictive

of the final results of the clinical trials or other trials; whether data from clinical trials will support submission for regulatory approvals; uncertainties as to the FDA s and other regulatory authorities interpretation of the data

from Dyne s clinical trials and acceptance of Dyne s clinical programs and as to the regulatory approval process for Dyne s product candidates; whether Dyne s cash resources will be sufficient to fund its foreseeable and

unforeseeable operating expenses and capital expenditure requirements; as well as the risks and uncertainties identified in Dyne s filings with the Securities and Exchange Commission (SEC), including the company s most recent Form 10-Q and in subsequent filings Dyne may make with the SEC. In addition, the forward-looking statements included in this press release represent Dyne s views as of the date of this press release. Dyne

anticipates that subsequent events and developments will cause its views to change. However, while Dyne may elect to update these forward-looking statements at some point in the future, it specifically disclaims any obligation to do so. These

forward-looking statements should not be relied upon as representing Dyne s views as of any date subsequent to the date of this press release.

1. DYNE-101 safety data as of April 23, 2025