This trend, previously reported, likely relates to two processes: (1) embryonic lethality of genomes containing 0 copies of SMN1 and less than 2 copies of SMN2, and (2) pathogenic SMA alleles often contain a conversion type mutation wherein the sequence of SMN1 is converted to that of SMN2, increasing SMN2 copy number in individuals with the less severe forms of SMA [34]–[36]. This evidence concerns the gene SMN1 and proximal spinal muscular atrophy.