In the mouse the MHC Class 1 loci are called H2K, H2L and H2D. The equivalent loci in the human MHC are HLA (Human Leukocyte Antigen). MHC class I molecules are heterodimers, consisting of a single transmembrane polypeptide chain (the a-chain) which is highly polymorphic, and the invariant β2 microglobulin (which is encoded elsewhere, not in the MHC). MHC class I molecules are found on almost every nucleated cell of the body. Their major function is to present peptide fragments derived from antigens to cytotoxic T cells.