Family studies indicate that the SMN2 gene is the main modifier of the SMA phenotype, where more SMN2 copies equate to a milder phenotype, but this observation is not absolute, and other modifiers exist both within and outside of the SMN2 region [9,11,12,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296]. The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.