Murine CD38, a type II transmembrane glycoprotein, is a bifunctional ectoenzyme capable of catabolizing nicotinamide adenine dinucleotide (NAD+) to cyclic ADP-ribose (cADPR) and then hydrolyzing cADPR to adenosine diphosphoribose (ADPR). It is expressed at high levels on the surface of peripheral B-lineage cells and at low density on germinal center B cells from unimmunized mice. It has also been reported to be expressed at moderate levels on NK cells, a proportion of peripheral T cells, and a subpopulation of thymocytes which are mostly TCRaß+, CD4-, CD8-. Murine CD38 is also expressed by all Mac-1+ macrophages in the peritoneal cavities of unimmunized mice, but not by unstimulated bone-marrow-derived macrophages. Monoclonal antibodies to CD38 have been shown to induce B and T cell proliferation, protect B cells from apoptosis, and inhibit B lymphopoiesis.