Background: A self-catalytic depurination sequence centered at codon 6 in the -globin gene creates a mutagenic apurinic site. Results: Unique codon 6 haplotypes, many anemia-and thalassemia-causing, far exceed haplotypes at other -globin codons. Conclusion: Excessive mutagenicity at the only -globin self-depurination site indicates a mechanism discovered in vitro that functions in vivo. Significance: In vivo functionality of self-depurination sites in genes can spontaneously cause diseases via somatic mutations.