The higher number of SMN2 copy number is associated with less severity of the SMA phenotypes (1 copy of SMN2: 96%–type I SMA; 2 copies: 79%–type I; 3 copies: 54%–type II and 31%–type III/IV; ≥ 4 copies: 88%–type III/IV) (2). This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.