CD38 (cluster of differentiation 38), also known as cyclic ADP ribose hydrolase is a glycoprotein found on the surface of many immune cells (white blood cells), including CD4⁺, CD8⁺, B lymphocytes and natural killer cells. CD38 also functions in cell adhesion, signal transduction and calcium signaling. In humans, the CD38 protein is encoded by the CD38 gene which is located on chromosome 4. CD38 is a multifunctional ectoenzyme that catalyzes the synthesis and hydrolysis of cyclic ADP-ribose (cADPR) from NAD+ to ADP-ribose. These reaction products are essential for the regulation of intracellular Ca²⁺.