SMN1 and proximal spinal muscular atrophy: 5q spinal muscular atrophy (SMA) is an autosomal-recessive neuromuscular disease caused by homozygous deletions or loss-of-function mutations in the survival motor neuron 1 gene (SMN1) with retained function of at least one copy of the paralogous SMN2 gene, resulting in a progressive loss of alpha motor neurons in the spinal cord and lower brain stem.