The death of the patient with a devastating muscle disease known as spinal muscular atrophy occurred in a late-stage trial in Europe that’s still enrolling patients, the Swiss drugmaker said in an emailed statement. Initial findings show it involved a severe respiratory infection followed by neurological complications. The results of an autopsy are pending, and details have been reported to regulators, the company said.

Novartis plans to introduce Zolgensma this year in the US and Europe and the report may cast doubt on a medicine viewed as revolutionary. A US regulatory decision is due by next month on the drug, intended to treat sick babies at high risk of death by their second birthday because of the muscle disease. The treatment is being closely watched as a test case for a burgeoning field: dozens of gene therapies that promise to fix debilitating and sometimes lethal flaws in critical genes are advancing toward the market.

Another piece of data on the Novartis gene therapy reported on April 16 raises concern. A patient died from respiratory failure in a study, though the investigator and an independent safety board concluded the event was unrelated to the treatment, according to a news release. The Swiss drugmaker said the study’s results backed the effectiveness of the therapy and demonstrated prolonged event-free survival and the achievement of key milestones.

Shares of Regenxbio Inc, which stands to receive milestones from Novartis for Zolgensma, fell 7.8% in US trading on Monday. Novartis rose 0.6% in Zurich on Tuesday to 77.50 Swiss francs.