Transgenic over-expression of SMN in the nervous systems of SMA mice effected almost complete rescue of the disease phenotype [14], but the degree of rescue attributable to expression within the motor neurons could not be unequivocally determined owing to the expression of the SMN transgene in all neurons as well as skeletal muscle, a second likely contributor to the SMA phenotype [15]–[17]. This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.