All clinical forms of SMA map to chromosome 5q13, a region that contains two survival motor neuron (SMN) genes that compromise two nearly identical copies, a centromeric copy of the survival motor neuron 2 (SMN2) gene and a telomeric copy of the survival motor neuron 1 (SMN1) gene [7]. This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.