CD91, also known as a2-macroglobulin receptor (a2MR1. Moestrup SK, et al. Cell Tissue Res 269:374-382, 1992. 2. Moestrup SK and Hokland P. Leuk Res 16(3):227-234, 1992. 3. Strickland DK, et al. J Biol Chem 265:17401-17404, 1990.), apolipoprotein E receptor (ApoER) and low density lipoprotein receptor-related protein (LRP), is a large two-chain membrane protein that binds the proteinase-activated form of the plasma membrane protein a2M.1 a2M is a proteinase-inhibitor containing a region that is a substrate for a wide range of proteinases including trypsin, chymotrypsin, plasmin, neutrophil elastase and fibroblast collagenase.1 Binding of a2M:proteinase complexes to a2MR is followed by endocytosis and degradation of the ligand in lysosomes. CD91 consists of two non-covalently associated polypeptides of ~420 to ~85kD, and has been shown to be present in a restricted spectrum of cell types, including neurons and astrocytes in the central nervous system, epithelial cells in the gastrointestinal tract, smooth muscle cells, fibroblasts, Leydig cells in testis, granulose cells in ovary, and dendritic interstitial cells of kidney.1 The high abundance of CD91 in certain cells types of most organs suggest two different mechanisms of CD91 -mediated ligand clearance.1 CD91 is expressed on malignant cells from patients with acute and chronic myelomonocytic leukemia, while no significant expression was found on malignant cells from acute and chronic lymphatic leukemia, lymphomas, plasma cell leukemias or hairy cell leukemia. CD91 was also shown to have a close correlation with CD14 surface expression.