MHC class I polypeptide-related sequence A/B (MICA/B) is a cell surface glycoprotein encoded by the MICA/B gene located within MHC locus. MICA and MICB are related to MHC class I and has similar domain structure, which is made up of external α1α2α3 domain, transmembrane segment and C-terminal cytoplasmic tail. MICA/B rather functions as a stress-induced ligand for NKG2D receptor. MICA is broadly recognized by NK cells, γδ T cells, and CD8+ αβ T cells which carry NKG2D receptor on their cell surface. As a result of NKG2D-MICA engagement, effector cytolytic responses of T cells and NK cells against epithelial tumor cells expressing MICA are initiated. MICA/B are not associated with β2-microglobulin nor binds peptides as conventional MHC class I molecules do. The heat shock stress pathway is involved in the regulation of MICA expression as transcription of MICA is regulated by promoter heat shock element.