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Hypothetical outline of the evolutionary tree of HLA. B2m-free HC of HLA-I or the proto-HLA is considered as the primitive variant of HLA-I and II, which by dimerization gives rise to homo- and heterodimers. B2m would have associated with monomers (Face-2) to evolve as HLA-I. Another independent monomer would have combined with modified Face-2 to evolve as HLA-II heterodimers.
