A young person with Duchenne muscular dystrophy died following treatment with the recently approved gene therapy delandistrogene moxeparvovec (Elevidys), Sarepta Therapeutics said in a statement Tuesday.

The young man died of acute liver injury, a known side effect, Sarepta said in a statement. But the company said the “severity” of the patient’s case had not previously been seen with the therapy, called Elevidys. It’s the first known patient death with the therapy, which has been used in more than 800 patients, the company said.

The patient, whom Sarepta described as a young man and later told news outlets was 16 years old, suffered acute liver failure after being treated with Elevidys, the only gene therapy that has won FDA approval as a treatment for DMD.