SMN2 and proximal spinal muscular atrophy: Only 20% of the total full-length SMN (FL-SMN) transcript is produced from the SMN2 gene, which partly compensates for the lack of FL-SMN transcript produced from SMN1 in patients with SMA, but does not produce sufficient SMN protein levels in motor neurons for their survival (Zheleznyakova et al., 2011).