THE products of the classical human major histocompatibility complex (MHC) class I genes (HLA-A, -B, -C) are highly polymorphic molecules that bind peptides and present them to T lymphocytes1-4. The non-polymorphic, non-classical MHC class I gene products (HLA-E, -F, -G) are not restricting elements for the majority of T lymphocytes5-7. The evolutionary relationship of the non-classical and classical MHC class I genes is unclear. Here we present the cloning and sequencing of the MHC class I genes of a New World primate, the cotton-top tamarin (Saguinus oedipus). The expressed MHC class I genes of this species are more closely related to the human non-classical HLA-G (ref. 7) gene than they are to genes of the human classical HLA-A, -B, and -C loci. These observations imply that classical and nonclassical genes do not necessarily constitute mutually exclusive groups over evolutionary time.
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