SMN2 and spinal muscular atrophy, type II: Patients with 2 copies of SMN2 most often develop SMA type 1 and less frequently type 2; patients with 3 copies of the SMN2 gene most commonly develop SMA type 2 but can also develop type 1 or type 3; and patients with 4 or more copies of the SMN2 gene usually develop type 3 or type 4 SMA.