Cyclic ADP-ribose (cADPR) is a Ca 2+ mobilizing second messenger found in various cell types, tissues and organisms. Receptor-mediated formation of cADPR may proceed via transmembrane shuttling of the substrate NAD and involvement of the ectoenzyme CD38, or via so far unidentified ADP-ribosyl cyclases located within the cytosol or in internal membranes. cADPR activates intracellular Ca 2+ release via type 2 and 3 ryanodine receptors.