The literature search was conducted on PubMed and Consensus (https://consensus.app/) using combinations of the keywords: “tryptophan metabolism,” “kynurenine,” “serotonin,” “indole,” AND each disorder name (e.g., “autism,” “ADHD,” “Rett syndrome,” “MeCP2 duplication”), with no time restriction and including both preclinical and clinical studies. The gene discussed is MECP2; the disease is Rett syndrome.