CD38 is a transmembrane glycoprotein found on immature cells of the B- and T-cell lineages but not on most mature resting peripheral lymphocytes. It is also present on thymocytes, pre-B-cells, germinal center B-cells, mitogen-activated T-cells, Ig-secreting plasma cells, monocytes, NK cells, erythroid and myeloid progenitors in the bone marrow, and brain cells.