We have identified a new A*66 allele (A*6603) in three related individuals, an Arabic patient suffering from acute myeloid leukemia and two of her relatives. The A*66 alleles differ in three amino acid residues at positions 70, 90 and 163. The closer relationship between A*6602 and A*6603, which only differ at amino acid 70, replacing GLN with HIS, suggests that the alloreactive potential in this mismatch combination is lower than in all other mismatched A*66 donor-recipient combinations, which exhibit two (A*6601 versus A*6602) and three (A*6601 versus A*6603) differences at the pivotal positions, respectively. This emphasizes the potential role of the A*66 subtypes in bone marrow transplantation with alternative donors. For that reasons, allelic subtyping should be considered in donor-recipient matching to identify the kind of disparity.
