The gene responsible for SMA is survival motor neuron 1 (SMN1), located in a ∼1.7 Mb tandemly duplicated region on chromosome 5q13.2 which also contains a duplicate gene SMN2. Homozygous loss of SMN1 due to deletion and/or conversion to SMN2 causes ∼95% of SMA cases, with the remaining 5% caused by compound heterozygosity of an SMN1 deletion and an SMN1 intragenic mutation (Wirth, 2000; D’Amico et al., 2011). The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.