Two survival motor neuron (SMN) genes on chromosome 5q13 have been correlated with SMA: telomeric SMN1 and centromeric SMN2. SMA is caused by deletions or loss-of-function mutations in SMN1 with the retention of SMN2 [5-8], resulting in production of insufficient full-length SMN transcripts. Here, SMN1 is linked to proximal spinal muscular atrophy.