Possible assortment of a1 and a2 region gene segments in human MHC class I molecules.

Using pair-wise comparison of aligned nucleotide sequences of distinct and complete human MHC class I molecules, we have constructed triangular tables to study the similarities and differences of various a1 (exon 2) and a2 (exon 3) region sequences. There are two HLA-A (A6901 and A6601) and 13 HLA-B (B4201, B8101, B4102, B4801, B4007, B4001, B4802, Dw53, B4406, B4402, B3901, B1514 and B3702) sequences that have identical a1 sequences with other known MHC class I molecules, while their a2 sequences are the same as those of different ones. Of these 15, A6901, B4001 and B4802 have previously been suggested as the results of recombination between A6801 and A0201, B4101 and B8101, and B4801 and B*3501, respectively. However, many other sequences can also be used to generate them by recombination. Furthermore, their reciprocal products have never been identified. Thus, gene conversion has subsequently been suggested as an alternative. Another possible genetic mechanism for generating these nucleotide sequence similarities can be assortment, or that some gene segments can be duplicated or multiplicated to be used in different human MHC class I molecules. Interestingly, this genetic mechanism is probably absent for the generation of different mouse MHC class I molecules.