BACKGROUND:  Weak blood group A and B phenotypes are correlated with ABO glycosyltransferases exhibiting single‐amino‐acid changes and/or C‐terminal modifications.STUDY DESIGN AND METHODS:  A healthy donor diagnosed as having weak A antigen expression and his two children were subjected to extensive ABO typing. HeLa cells were used to transfect ABO expression plasmids.RESULTS:  The donor's red blood cells were type AweakB and his serum sample contained weakly reactive anti‐A1 antibodies. A single T>C transition identified at the +2 position of the start codon of an ABO*A101 allele predicted the disruption of this methionine codon. In the transfection studies, a significant reduction of A activity was observed on HeLa cells transfected with a plasmid containing the variant ABO*A allele. Coexpression of the respective antithetical ABO*B101 wild‐type construct further reduced cell surface A antigen expression. Similar expression results were obtained with ABO constructs in which the Met1 start codon and five alternative start sites at codons 20, 26, 43, 53, and 69 had successively been interrupted.CONCLUSION:  The donor's weak blood group A phenotype most likely resulted from expression of an N‐truncated A transferase triggered by alternative translation start sites in the transmembrane domain or stem region.