The initial hypothesis on the selective degeneration of motor neurons in SMA was based on the role of the SMN complex in the assembly of spliceosomes and pre-mRNA splicing, and it was thought that defects in the splicing of neuron-specific genes may contribute to selective degeneration (Gubitz et al., 2004; Monani, 2005; Burghes and Beattie, 2009). This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.