SMN1 and proximal spinal muscular atrophy: The survival motor neuron (SMN) gene, implicated in the four forms of SMA, maps to chromosome 5q 11.2–13.3 [5–7] and is duplicated as telomeric and centromeric copies, so called SMN1 (OMIM No. 600354) and SMN2 (OMIM No. 601627), respectively [8, 9].