CD38 (also referred to as T10 antigen) is a nonlineage-restricted type II transmembrane glycoprotein that has emerged as an intracellular calcium ion mobilizing messenger. It can serve as an ectoenzyme that catalyzes the synthesis and hydrolysis of cyclic ADP-ribose. The enzymatic functions of CD38 probably contribute to an array of its immunoregulatory functions. It has been found on the surface of many immune cells (white blood cells), including CD4+, CD8+, B lymphocytes and natural killer cells. Soluble CD38 and the ability of membrane-bound CD38 to become internalized in response to appropriate stimuli suggest that extracellular and intracellular roles for this protein are equally plausible.