Concretely, the higher number of SMN2 copies, the milder the SMA phenotype, as most patients comply with the following rule: SMA type I patients had 2 SMN2 genes, type II had 3 SMN2, type III had 3 or 4 SMN2 copies and type IV patients had 4 SMN2 genes [15]. This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.