SMN1 and proximal spinal muscular atrophy: Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder characterized by progressive muscle wasting due to motor neuron degeneration, secondary to mutations in the survival motor neuron 1 (SMN1) gene.1 SMA is classified according to age at onset and maximal motor functional status achieved: weak infants unable to sit unsupported (type 1), nonambulant patients (type 2), and ambulant patients with childhood (type 3) and adult (type 4) onset.2,3