SMN2 and spinal muscular atrophy: Spinal muscular atrophy (SMA) is a rare, debilitating neuromuscular disease, resulting in progressive muscle weakness and atrophy caused by a homozygous deletion or mutation of the Survival Motor Neuron 1 (SMN1) gene, which results in the loss of SMN protein expression and the subsequent degeneration of spinal cord and brainstem motor neurons.