Any of a class of histocompatibility proteins encoded by the major histocompatibility complex that are expressed by all nucleated cells. Following synthesis in the endoplasmic reticulum, these proteins bind peptide fragments derived from viruses or other pathogens invading the cytoplasm and are transported to the outer membrane of the cell, effectively flagging the cell for destruction by cytotoxic T cells bearing CD8 proteins. Each MHC class I protein consists of two polypeptide chains—an α‎ chain and β‎₂-microglobulin—and forms a peptide-binding cleft in which a short antigenic peptide, usually 8–10 residues long, becomes stably integrated. Tissue grafts from a mismatched donor are marked for rejection by the recipient’s immune system by virtue of the nonself MHC class I proteins carried by all tissue cells.