MHC class I polypeptide–related sequence A (MICA) is a highly polymorphic cell surface glycoprotein encoded by the MICA gene located within MHC locus. MICA functions as a stress-induced ligand (as a danger signal) for integral membrane protein receptor NKG2D ("natural-killer group 2, member D"). MICA is broadly recognized by NK cells, γδ T cells, and CD8+ αβ T cells which carry NKG2D receptor on their cell surface and which are activated via this interaction. Engagement of NKG2D-MICA results in activation of effector cytolytic responses of T cells and NK cells against epithelial tumor cells (or other stressed cells) expressing MICA on their surface. High levels of MICA in the serum of tumor patients are positively related to tumor size and poor prognosis. Variations in the MICA gene are also associated with susceptibility to psoriasis 1 and psoriatic arthritis and MICA-specific antibodies or its shedding are involved in the monoclonal gammopathy of undetermined significance´s (MGUS) progression to multiple myeloma.