SMA is thus 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 (reviewed in Burghes and Beattie, 2009; Monani and De Vivo, 2014). The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.