SMN1 and proximal spinal muscular atrophy: The disease course of SMA is characterized by highly variable severity and age of onset, which is caused in large part by differences in copy number of SMN2, a duplicate of SMN1 that can attenuate SMN1 loss (Arnold et al., 2016; Bharucha-Goebel and Kaufmann, 2017; Garcia-Lopez et al., 2018).