Loss of SMN1 is essential to the pathogenesis of SMA, while the severity of the disease is primarily related to the number of copies of SMN2. Most SMA type I patients have two copies of SMN2 [22], three SMN2 copies are common in SMA type II, while type III and IV generally have three or four [23,24]. The gene discussed is SMN2; the disease is spinal muscular atrophy, type II.