The SMN2 copy number and the resulting functional amount of SMN protein define the onset and severity of SMA, which can be replicated in the severe SMNΔ7 and intermediate Smn2B/– SMA mouse models (Le et al., 2005; Bowerman et al., 2012; Tisdale and Pellizzoni, 2015). Here, SMN1 is linked to proximal spinal muscular atrophy.