Glucose-6-phosphate dehydrogenase (G6PD) is remarkable for its genetic diversity in humans. Many variants of G6PD have been described with wide ranging levels of enzyme activity and associated clinical symptoms. Fifty-eight different mutations have now been identified and these account for 97 named G6PD variants. The mutations are almost exclusively missense mutations, causing single amino acid substitutions. They are spread throughout the coding region of the gene, although there appears to be a cluster of mutations that cause a more severe clinical phenotype towards the 3' end of the gene. The absence of large deletions, frameshift mutations and nonsense mutations is consistent with the notion that a total lack of G6PD activity would be lethal.