MHC class I polypeptide-related sequence A is a protein that in humans is encoded by the MICA gene.MICA encodes the highly polymorphic MHC (HLA) class I chain-related gene A. The protein product is expressed on the cell surface, although unlike canonical class I molecules does not seem to associate with beta-2-microglobulin. It is thought that MICA functions as a stress-induced antigen that is broadly recognized by NK cells, NKT cells, and most of the subtypes of T cells. MICA is a ligand for NK cell activating receptor NKG2D.