CD38 is a type II transmembrane glycoprotein of 45 kDa with a protein core of 35 kDa. The CD38 antigen is expressed on essentially all pre-B lymphocytes, plasma cells, and thymocytes. It is also present on activated T lymphocytes, natural killer (NK) lymphocytes, myeloblasts, and erythroblasts. The antigen is expressed during the early stages of T- and B-lymphocyte differentiation, is lost during the intermediate stages of maturation, and then reappears during the final stages of maturation. The CD38 antigen is expressed on 90% of CD34+ cells, and is not expressed on pluripotent stem cells. Coexpression of CD38 antigen on CD34+ cells indicates lineage commitment of those cells. CD38 is a counter-receptor of CD31. It is also expressed in T- and B-acute lymphoblastic leukemia (ALL), Burkitts lymphoma, multiple myeloma, acute myeloid leukemia (AML), and chronic lymphocytic leukemia (CLL). The OKT10 directed against human CD38 antigen.