The SMN2 gene, a centromeric paralog of SMN1, is the main genetic modifier of the SMA phenotype and disease severity: multiple copies of SMN2 may be present in the genome (up to 5 or 6 in SMA type 3 and 4), with a higher number inversely correlating with disease severity (10, 11). This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.