SMN2 and proximal spinal muscular atrophy: The survival motor neuron (SMN) gene is the disease-causing gene of SMA [1], and it exists as two nearly identical copies, SMN1 and SMN2. SMN1 is the critical gene involved in SMA, as more than 90% of SMA patients have SMN1 exon 7 homozygous deletions [1-3].