(From the link above): Spinal muscular atrophy (SMA) is a rare genetic disease caused by the deletion or mutation of the survival motor neuron 1 (SMN1) gene. The SMN1 gene produces survival motor neuron (SMN) protein that is critical for normal function of motor neurons.
Patients with SMA have an insufficient amount of SMN protein, which leads to permanent loss of motor neurons. Untreated, SMA Type 1 is the number one genetic cause of infant death.