Regardless of the details, an outstanding issue is that the physiological route of NAADP synthesis has still not been "unequivocally" identified — neither the reaction(s) nor the enzyme(s). Clearly, it is theoretically possible there may be multiple routes of synthesis, but this would be unprecedented in the second messenger world. To date, the most favoured hypothesis is the so-called "base-exchange reaction" (nicotinic acid + NADP → NAADP + nicotinamide; catalyzed by ADP-ribosyl cyclases) which are a family of enzymes that include CD38 and CD157 in mammals (and orthologs in sea urchin and "Aplysia" ovotestis). These were first discovered as the synthetic enzymes for cADPR but later revealed to be multifunctional, promiscuous enzymes that can also produce NAADP. Certainly NAADP production can occur "in vitro" but whether it occurs "in vivo" is another question (because genetic knockout or knock-down of ADP-ribosyl cyclases has no effect on NAADP production in some cell types), and there may be other routes which require different substrates and enzymes.