SMN2 and spinal muscular atrophy, type IV: Next, we determined the SMN2 copy number in 204 out of 221 SMA patients with homozygous SMN1 deletion (83 out of 93 patients with SMA type I, 70 out of 71 patients with SMA type II, 40 out of 46 patients with SMA type III, and 11 out of 11 patients with SMA type IV).