This null mutant, Smn−/−, served to highlight SMN as a developmentally essential protein since its complete ablation is embryonic lethal (Schrank et al., 1997) The heterozygous Smn+/− however, does not develop the typical histopathological SMA hallmarks and remains at best a hypomorphic model for the disease (Schrank et al., 1997; Bowerman et al., 2014). This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.