Spinal muscular atrophy is an inherited neuromuscular disorder. The gene responsible for the disease has been identified and named the SMN gene. This review is prompted by recent advances in understanding cellular function of the SMN gene and its gene product and by the increasing evidence that maturation of all parts of the neuromuscular system is delayed in spinal muscular atrophy patients. We suggest that the timing of developmental changes in motoneurons and muscles is critical for their survival. Delayed maturation of either motoneuron or muscle can cause these cells to die so the molecules that are involved in controlling their rate of maturation are crucial for normal development. We suggest that SMN gene/protein is one such molecule, because the neuromuscular system develops more slowly in spinal muscular atrophy patients, where SMN protein is absent, and in animals models, where SMN protein is reduced.