Deficiency of functional SMN protein results in spinal muscular atrophy (SMA), a genetic disorder characterized by selective degeneration of motor neurons and progressive paralysis (Lefebvre et al. 1995; Gubitz et al. 2004; Iannaccone et al. 2004; Pellizzoni 2007; Chari et al. 2009). Here, SMN1 is linked to proximal spinal muscular atrophy.