The limited full-length SMN protein expressed by SMN2 is not sufficient to compensate for the deficit of SMN1. SMA is classically subdivided into four clinical types based on the severity and age of onset: Type I is most severe, type II is the intermediate form, type III is the mild form and type IV is the adult-onset form5. The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.