Cyclic ADP-ribose hydrolase 1 (CD38) is a transmembrane protein involved in several important biological processes, including immune response, insulin secretion, and social behavior. Originally described as a glycosylated immune cell surface marker, additional research determined that CD38 is a multifunctional enzyme that catalyzes the synthesis and hydrolysis of cyclic ADP ribose (cADPR) from NAD. Under acidic conditions, CD38 also catalyzes the synthesis of nicotinic acid adenine dinucleotide phosphate (NAADP) from NADP+. Both cADPR and NAADP act as calcium ion mobilizing messengers that target different intracellular Ca2+ stores. Since CD38 is the primary mammalian NAD+ glycohydrolase responsible for NAD+ metabolism, CD38 may be a valuable therapeutic target for treatment of metabolic diseases regulated by NAD+-dependent pathways. CD38 has also been considered a possible therapeutic target for antibody-mediated therapy for myeloma and chronic lymphocytic leukemia.