Spinal muscular atrophy is an autosomal recessive disease characterized by motor neurone loss, muscle atrophy and weakness. Deletion or mutation of the SMN1 gene reduces intracellular survival motor neurone protein levels causes spinal muscular atrophy, most likely by interfering with spliceosome assembly. A range of clinical severity and corresponding survival motor neurone levels is seen because of the presence of copies of the transcriptionally inefficient SMN2 gene and possibly other modifying genes. The delineation of SMN1 as the gene that causes spinal muscular atrophy and the identification of genes that modify spinal muscular atrophy raise the prospect of gene therapy or in-vivo gene activation treatment for this frequently fatal disorder.