AANAT (Aralkylamine N-acetyltransferase), also called Serotonin N-acetyltransferase (SNAT) or Serotonin acetylase, is a member of the acetyltransferase superfamily. It is almost exclusively expressed in the pineal gland and the retina. AANAT activity is high at night and low during the day due to the 10 to 100-fold increase in the amount of active AANAT at night. Retinal exposure to light causes cAMP levels to decrease in photoreceptor cells and, as a result, AANAT is targeted for degradation by proteasomal proteolysis. AANAT plays an important role as the rate limiting enzyme in melatonin synthesis. It is responsible for catalyzing the N-acetylation of serotonin to N-acetylserotonin, which is then converted to melatonin by hydroxyindole-O-methyltransferase. Melatonin is an important hormone that is involved in many physiological processes including immune function, seasonal reproduction, retinal physiology and circadian entrainment.