SMN1 and proximal spinal muscular atrophy: SMN1 deficiency or dysfunction in motor neuron-like cells is associated with loss of growth cones, structural defects in axons that impair pathfinding and innervating abilities of these neurons, and contributes to Spinal Muscular Atrophy (SMA) pathogenesis (Allegri et al., 2010; Du et al., 2008; Tunbridge et al., 2006).