The clone OKT10, a mouse monoclonal antibody, recognizes a 45 kDa, type II transmembrane cell surface glycoprotein known as CD38. The CD38 antigen is expressed on pre-B lymphocytes, plasma cells, thymocytes and monocytes. CD38 also presents on activated T lymphocytes, natural killer (NK) lymphocytes, myeloblasts, and erythroblasts. Antigen expression is detected during the early stage of T- and B-lymphocyte differentiation, then lost during the intermediate stage of maturation, only to reappear during the final stage of maturation. The CD38 antigen is expressed on 90% of CD34+ cells and is not expressed on pluripotent stem cells. CD38 is expressed in T- and B-acute lymphoblastic leukemia (ALL), Burkitt's lymphoma, multiple myeloma, acute myeloid leukemia (AML), and chronic lymphocytic leukemia (CLL).