SMN1 and proximal spinal muscular atrophy: It has been hypothesized that the mechanism which leads to SMA is either a loss-of-function of SMN-mediated snRNP assembly role, resulting in alternative splicing of target genes (Gabanella et al., 2007) or a loss-of-function of mRNA transport in neurons (Fan and Simard, 2002; Eggert et al., 2006).