CD38 is a single chain type II integral transmembrane protein, which is highly expressed on thymocytes. It is also present on activated T cells and terminally differentiated B cells (plasma cells). Other reactive cells include NK cells, monocytes, macrophages, and dendritic cells. CD38 may be detected on cells from multiple myeloma, ALL (B and T) and some AML. It is, however, not found on most mature resting peripheral lymphocytes. CD38 functions as a multicatalytic ectoenzyme serving as ADP-ribosyl cyclase, cyclic ADP- ribose hydrolase and possibly NAD+ glycohydrolase or as a cell surface receptor. Antibodies to CD38 are useful in subtyping of lymphomas and leukemias, detection of plasma cells (i.e. identification of myelomas), and as a marker for activated B and T cells.