SMN1 and proximal spinal muscular atrophy: SMA is caused by loss-of-function mutations of the SMN1 (survival of motor neuron 1) gene located on chromosome 5q13, and more than 95 percent of SMA patients show homozygous deletion of exon 7 of the SMN1 gene, leading to a reduction in survival motor neuron (SMN) protein and motor neuron dysfunction [4].