Recent studies highlight a bidirectional relationship between melatonin, synthesized via the tryptophan–serotonin–N-acetylserotonin (NAS)–melatonin pathway (involving arylalkylamine N-acetyltransferase (AANAT) and acetylserotonin-O-methyltransferase (ASMT) enzymes [7,8,9]), and gut microbial composition, with dysregulation linked to breast cancer (BC) development through altered tryptophan metabolism toward the kynurenine pathway [10,11]. The gene discussed is ASMT; the disease is breast carcinoma.