B2M / Beta 2 Microglobulin is a serum protein associated with major histocompatibility complex (MHC) class I heavy chain on the surface of nearly all nucleated cells. It is required for proper cell surface expression of the complex. The protein has a predominantly beta-pleated sheet structure that can form amyloid fibrils in some pathological conditions. In cancer, mutation, loss or deregulation of B2M is believed to be involved in aiding tumors in escaping the immune response and therapies. It is used as a marker in the diagnosis of multiple myeloma and lymphoma, where it is regularly overexpressed. It has elevated levels in a number of other cancers, including colorectal, breast, gastric, lung, prostate and testicular cancer as well as malignant epithelial ovarian tumors. B2M has membranous and cytoplasmic positivity in most normal tissues, with elevated levels in immune cells and macrophages. References: J Transl Med. 2016; 14: 75, PMID: 26983758; Cancer Discov. 2017 Dec; 7(12): 1420–1435, PMID: 29025772