SMN1 and proximal spinal muscular atrophy: SMA is an autosomal recessive neuromuscular disorder, and ~95% of SMA patients have a homozygous deletion of the SMA-causative gene, SMN1. Thus, SMA carriers are usually diagnosed based on their SMN1 copy number, because one SMN1 copy means the heterozygous deletion of SMN1 ([1 + 0] genotype).