Spinal muscular atrophy (SMA), a childhood-onset motor neuron disease, has historically been the most frequent genetic cause of infant mortality,1 although this is likely to change with the recent therapeutic “revolution.” SMA, caused by mutations in the Survival Motor Neuron 1 (SMN1) gene, leads to loss of SMN protein expression. The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.