A young boy died in a trial for Pfizer’s experimental gene therapy for Duchenne muscular dystrophy, the company told patient advocates Tuesday. The boy was enrolled in Daylight, a trial studying the treatment in boys aged 2 or 3. The boy had received the therapy early last year, Pfizer told the advocates in a note posted online by Parent Project Muscular Dystrophy. Pfizer said it had not yet determined precisely what happened or whether it was linked to the treatment. Deaths and severe side effects linked to gene therapy have generally happened soon after dosing.