ASMT (acetylserotonin O-methyltransferase) is the final enzyme in the melatonin biosynthesis pathway, catalyzing the conversion of N-acetylserotonin to melatonin 1. The enzyme functions as a physiologically active dimer with distinct N-terminal and C-terminal domains, where the N-terminal domain facilitates dimerization and the C-terminal domain performs SAM-dependent O-methyltransferase activity 1. ASMT is located on the X chromosome X|Y exhibits sex-specific effects, particularly in females where it maintains gut microbiota plasticity and enhances neurobehavioral adaptability to exercise 2. The enzyme's dysfunction is clinically significant across multiple neuropsychiatric conditions. Deleterious ASMT mutations reduce enzyme activity and are associated with autism spectrum disorders, intellectual disability, and bipolar disorder 345. In depression, both mRNA and protein expression levels are significantly decreased and correlate with cognitive impairment 6. ASMT variants affect circadian rhythms and sleep patterns, with functional polymorphisms influencing sleep duration and activity patterns 5. The enzyme's role extends beyond neuropsychiatry, as disrupted ASMT activity and altered NAS/melatonin ratios have been implicated in hormone receptor-positive breast cancer pathophysiology 7.