SMN1 and proximal spinal muscular atrophy: Homozygous deletion of exon 7 or other mutations in the SMN1 gene on chromosome 5q13, resulting in survival motor neuron (SMN) protein deficiency (1), causes classic proximal spinal muscular atrophy (SMA), one of the most frequently occurring neuromuscular diseases with an incidence of about 1/10,000 live births (2).