The antisense oligonucleotide nusinersen is safe and tolerable, and might improve motor function in children with spinal muscular atrophy (SMA), according to a new phase I trial. SMA is caused by mutations in the SMN1 gene, and nusinersen compensates for loss of SMN1 function by altering the splicing of the mRNA encoded by the paralogue SMN2. Trial participants who received a single 9 mg intrathecal injection of nusinersen showed significant increases in motor scores both 3 months and 9–14 months after treatment, and the results indicate that the drug warrants further investigation in patients with SMA.