Key Points SLC29A1 encoding the equilibrative nucleoside transporter 1 (ENT1) specifies a novel blood group system that includes the Ata antigen. Although At(a−) people of African ancestry have functional ENT1, 3 siblings of European ancestry were identified who do not express ENT1.

BACKGROUND:  In the ABO blood group system mutations in the A gene may lead to weak A subgroups owing to a dysfunctional 3‐α‐N‐acetylgalactosaminyltransferase. STUDY DESIGN AND METHODS:  Blood and DNA were investigated to correlate weak A phenotypes with genotype, and an overrepresentation of the infrequent O2 allele was observed. Consequently, 57 available O2 alleles were examined in detail. RESULTS:  Two new O2 alleles were identified having mutations resulting in Gly229Asp with or without Arg217Cys. A recently described O2 variant (488C>T; Thr163Met) was also found. Surprisingly, both the original and the variant O2 alleles were associated with either O or Aweak phenotypes. Three novel O alleles surfaced in six other samples with suspected A subgroups. These were A1‐like alleles having nonsense mutations causing premature truncation at codons 56, 107, or 181. A second example of the rare O3 allele was also identified. A newly described O1 allele having 768C>A was found to be the third most frequent O allele among Swedish donors. Of the five novel O alleles, three were incorrectly interpreted as A1 following routine ABO genotyping. CONCLUSION:  Apparent O alleles lacking 261delG may cause weak A expression on red blood cells and/or inhibit anti‐A production. A hypothesis that exchange of genetic material between principally dissimilar O alleles during mitosis (“autologous chimerism”) restores glycosyltransferase activity in some cells would explain this interesting phenomenon.