SMN1 and proximal spinal muscular atrophy: While SMA is caused by biallelic mutations in the Survival Motor Neuron 1 (SMN1) gene, severity of disease is largely determined by the copy number of the paralogous gene SMN2. SMN2 differs by 5 nucleotides from SMA1; which leads to alternative splicing reducing the amount of functional SMN protein to ~10% compared to SMN1 (2).