The rarity of spinal muscular atrophy (SMA) means that promising new treatments may be tested in only a limited spectrum of patients before approval. Investigators evaluated a newly approved drug, onasemnogene abeparvovec, in a broader spectrum of patients in order to obtain expanded data on its side effects profile. They report in the Journal of Neuromuscular Diseases that the drug is associated with an immune response against the adeno-associated viral vector and needs careful monitoring, but showed no long-term adverse effects.