SMN2 and proximal spinal muscular atrophy: SMN1 is located on chromosome 5q13 and is one of two genes encoding the SMN protein.1,2 The incidence of SMA in Europe is about 1/10,000 livebirths with a carrier frequency of 1/50.3 Based on the onset of clinical symptoms, the achievement of motor milestones, and life expectancy, SMA is divided into four subtypes (SMA type 0–4) according to the International SMA Consortium.