SMN1 and proximal spinal muscular atrophy: 5q spinal muscular atrophy (SMA) is an autosomal-recessive motor neuron disease caused by the bi-allelic loss of function of the survival of motor neuron 1 (SMN1) gene on chromosome 5q13 with at least one functional copy of the paralogous survival of motor neuron 2 (SMN2) gene.