SMN1 and proximal spinal muscular atrophy: Humans harbor two genes encoding SMN, SMN1 and SMN2. In SMA patients SMN production is drastically reduced by homozygous deletion/mutation of SMN1. In such patients SMN is solely derived from SMN2. SMN2 is essentially identical to SMN1 except for a C→T nucleotide change that causes exon 7 skipping in the splicing of ∼90% SMN2 RNA, leading to the synthesis of an unstable, minimally functional protein (SMNΔ7).