Big pharma giant Pfizer has been forced to pause its trial of a drug for a rare muscle-wasting disease after a child who received it died suddenly.

“A fatal serious adverse event was reported as cardiac arrest for a participant in the Phase 2 DAYLIGHT study,” the company revealed in an online letter.

They said the participant, a young boy suffered acardiac arrest and died after receiving Pfizers one time treatment last year. He was part of a mid-stage study, now concluded, that enrolled children between the ages of 2 and 4 years old.

NDTV reports: Pfizer has paused administering the same gene therapy in a separate, final-stage study while it investigates the boy’s death, the company said. That study is looking at the gene therapy, called fordadistrogene movaparvovec, to treat Duchenne muscular dystrophy, a rare and fatal muscle-wasting disease that primarily affects boys.

Pfizer said it’s working to “evaluate our investigational gene therapy while protecting the safety of the participants, which is our top priority.”

The final-stage trial enrolled boys between the ages of 4 and 8, testing whether the therapy could slow disease progression compared to a placebo. Results from that study are expected this year.