SMN2 and proximal spinal muscular atrophy: In SMA patients, more than 95% of cases have homozygous deletion of the SMN1 gene; even if patients have SMN2, the majority of the gene products are SMN-Δ7 which is non-stable and degrades quickly, and thus could not compensate for the loss of SMN1 (Pearn, 1980; Lefebvre et al, 1997).