SMN1 and proximal spinal muscular atrophy: The SMN gene is duplicated as an inverted repeat on each allele of human chromosome 5, typically existing as a telomeric copy of SMN (SMN1), which is deleted or mutated in over 98% of SMA patients [2], and at least one copy of the centromeric SMN (SMN2), which provides the basis for survival in the absence of SMN1 [3].