SMN2 and proximal spinal muscular atrophy: Spinal muscular atrophy (SMA) is a progressive autosomal recessive motor neuron disease caused by a deficiency of survival motor neuron (SMN) protein due to biallelic deletion or a pathogenic variant of the SMN1 gene [1], which is classified into five types on the basis of onset age and severity, and the severity generally correlates inversely with the copy number of SMN2, a homologous gene on chromosome 5q13 [2].