SMN1 and proximal spinal muscular atrophy: Residual intact SMN translated from each SMN2 copy partially compensates for SMN1 deficiency such that genomic SMN2 copy number correlates inversely with disease severity [9–11]: two SMN2 copies commonly segregate with a severe (type 1) SMA phenotype whereas three or more copies correlate with later disease onset and slower progression [12, 13].