SMN1 and proximal spinal muscular atrophy: SMN1 and SMN2 present on chromosome 5q13, and of the 5q13-linked SMA patients, 96.4% show homozygous absence of SMN1 exons 7 and 8 or exon 7 only, whereas 3.6% present a compound heterozygosity with a subtle mutation on one chromosome and a deletion/gene conversion on the other chromosome [6].