SMN1 and proximal spinal muscular atrophy: Human models of SMA have been established by generating induced pluripotent stem cells (iPSCs) from individuals with SMA (Chang et al., 2011; Corti et al., 2012; Ebert et al., 2009; Sareen et al., 2012) and knocking down the functional SMN in hESCs (Wang et al., 2013), which recapitulate disease-specific degeneration in motor neurons.