There is a copy number polymorphism (CNP, variability of the total number of copies of functional genes between individual genomes) of both genes, and a tendency for patients with a higher number of SMN2 copies to develop milder forms of SMA; for example, patients with type 1 SMA usually have no more than two copies of SMN2 [7], whereas patients with type 2 often have three copies of SMN2 [8], and so on and so forth. The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.