SMA patients have a homozygous loss of the survival motor neuron 1 (SMN1) gene, but retain one or more copies of a nearly identical homolog, SMN2. Therefore this disease is the result of insufficient amounts of SMN protein and its levels are generally inversely correlated with the severity of the disease, hence making SMN2 copy number the predominant modifier of the neuromuscular phenotype (Monani and De Vivo, 2014). The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.