['Whether MHC molecules undergo concerted evolution or not has been the subject of a long-standing debate.', 'By comparing sequences of eight functional homologues of HLA-E from primates and rodents with those of MHC class Ia molecules from the same eight species, we find that different portions of MHC class I molecules undergo different patterns of evolution. By focusing our analyses sequentially on these various portions, we have obtained clear evidence for concerted evolution of MHC class I molecules, suggesting the occurrence of extensive interallelic and intergenic exchanges. Intra-species homogenisation of sequences is particularly noticeable at the level of exon 4, which codes for the alpha3 domain, but our results suggest that homogenisation also concerns certain residues of the alpha1-alpha2 codomain that lie outside the antigen recognition site.'] Is the orthology of HLA-E and H2-Qa1 hidden by their concerted evolution with other MHC class I molecules?