AANAT (aralkylamine N-acetyltransferase) is the rate-limiting enzyme in melatonin biosynthesis, catalyzing the N-acetylation of serotonin to N-acetylserotonin, the penultimate step in melatonin synthesis 1. Located on chromosome 17, AANAT controls the night/day rhythm of melatonin production in the pineal gland and is highly expressed in pineal tissue, with lower expression in retina, brain regions, and pituitary gland 1. The enzyme's activity is regulated by circadian mechanisms and can be modulated by inflammatory mediators like interleukin-6 and interleukin-18, which inhibit AANAT gene transcription 2. AANAT expression is also regulated by SIRT6, with dysregulation leading to impaired tryptophan catabolism and neurotoxic metabolite accumulation 3. Clinically, AANAT gene polymorphisms show significant associations with major depressive disorder, particularly in young males 45, and with seasonality patterns in bipolar disorder 6. The enzyme represents a potential therapeutic target for circadian rhythm disorders and conditions involving abnormal melatonin levels, such as seasonal affective disorder 7. AANAT's role in melatonin production makes it crucial for regulating sleep-wake cycles, seasonal behaviors, and potentially mood-related disorders 8.