Cambridge, Mass.-based Sarepta Therapeutics reported the first known death from its Duchenne muscular dystrophy gene therapy treatment. The patient was a young man who suffered acute liver failure after being treated with Elevidys,

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.