The genetic defect in 95% of cases is homozygous deletion of theSMN1 gene and while theSMN2 gene, present in all patients with SMA, potentially encodes an identical protein, a synonymous C>T variant in exon 7 leads to aberrant processing of the mRNA.SMN2 copy number is polymorphic, and there is some inverse correlation between the number of copies ofSMN2 and disease severity. This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.