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). Here, SMN1 is linked to proximal spinal muscular atrophy.