SMN1 and proximal spinal muscular atrophy: In healthy individuals, the SMN1 gene can compensate for overall SMN protein production; however, in more than 90% of SMA cases, there is a homozygous deletion or mutation in the SMN1 (Lefebvre et al., 1995; Farrar and Kiernan, 2015) gene, resulting in an inability of SMN1 to counterbalance SMN protein levels.