Despite the homozygous deletion in SMN1, patients with SMA are viable because of the existence of insufficient intrachromosomal duplications of SMN1, the SMN2 gene, centromeric to SMN1. However, a base pair substitution in exon 7 and the intron silence sequence in intron 7 of SMN2 lead to an altered splicing, and consequently exon 7 is missing in ~90% of final mRNAs. The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.