SMN1 and proximal spinal muscular atrophy: Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease that leads to progressive muscle wasting due to the selective loss of spinal cord motor neurons (MNs) caused by mutations in the survival of motor neuron 1 gene (SMN1) (Crawford & Pardo 1996; D'Amico et al. 2011; Lefebvre et al. 1995).