The death of a 51-year-old man in the study follows two other deaths of Duchenne patients treated with Sarepta’s marketed gene therapy Elevidys.

At the FDA's request, delandistrogene moxeparvovec (Elevidys), the only approved gene therapy for Duchenne muscular dystrophy, will carry a black box warning for acute liver injury and acute liver failure, drugmaker Sarepta Therapeutics said.

Shares of Sarepta Therapeutics (NASDAQ: SRPT) plunged 26.85% in Friday pre-market after a clinical trial participant died following treatment with a gene therapy, marking the third death linked to the company's experimental treatments.

Sarepta Therapeutics to lay off 493 workers, including 80 in Ohio, amid FDA probe, stock plunge, and concerns over Duchenne gene therapy Elevidys.

Shares of Sarepta Therapeutics declined 17% in early trading on Friday after another patient who had received an experimental gene therapy died, deepening investor concerns over the use of the company's treatments.

The restructuring enacted by Sarepta Therapeutics is expected to save up to $400 million in annual costs. Read why I'm downgrading SRPT stock from Hold to Sell.

Three patients with a muscle-wasting disease died from liver failure after taking the therapy, Ele­vidys, or a similar treatment.