Involved in the presentation of foreign antigens to the immune system. Heterodimer of an alpha chain and a beta chain (beta-2-microglobulin). The antigen encoded by the PAG64 clone, normally expressed only on lymphocyte subsets and thymocytes, is found in increased concentrations in all transformed cells tested, in an embryonic carcinoma cell line, and in pluripotent embryonic cells. Activation of the gene appears to be a general feature of carcinogenesis in the mouse. The PAG64 clone, and related cDNA clones, have in common a repetitive sequence in their 3' transcription units that has characteristics of a transposable element. Exon 7 is missing. Belongs to the MHC class I family.