SMN2 and proximal spinal muscular atrophy: SMA is caused by a homozygous loss of function of survival motor neuron (SMN1 [MIM 600354]), which encodes for SMN, a key protein in the biogenesis of small ribonucleoparticles (snRNPs), which has recently been associated with a complex system of neuronal circuitry18–20 and has also been linked to transcription, stress response, apoptosis, axonal transport, and cytoskeletal dynamics.21–23 However, humans possess a SMN1 paralog, named SMN2 (MIM 601627), located as well on chromosome 5, but in centromeric position.