The data provide evidence that nondeletional ABO*O03-like alleles produce detectable amounts of A antigens.

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.

The frequency of variations in the CBF/NF-Y region of the ABO gene in these samples with presumably common ABO*B101 alleles suggests that weak blood group B phenotypes may be caused by sequence variations in the CBF/NF-Y regulatory region.