The GT6 glycosyltransferases gene family, which includes the ABO blood group (ABO; MIM 110300) and GLT6D1, shows a complex evolution pattern, with multiple events of gain and loss in different mammal species. In humans, the ABO gene is considered the sole functional member, although the O allele is null and is fixed in certain populations (summary by Casals et al. (2009) [PubMed 19218399]).[supplied by OMIM, Jan 2011].