SMN1 and proximal spinal muscular atrophy: Humans have two copies of the gene encoding SMN: SMN1 and SMN2. 95% of SMA cases are caused by homozygous deletion of SMN1. SMN1 and SMN2 are 99.9% identical, with just one base pair difference (Lorson et al., 1999), resulting in skipping of exon 7 in ∼85%–90% of SMN2 transcripts.