CNS—Central Nervous System; ENS—Enteric Nervous System; ASD—Autism Spectrum Disorders; GABA—Gamma-aminobutyric acid; SCZ—Schizophrenia; The HPA axis—The hypothalamic-pituitary-adrenal axis; BDNF—Brain-derived Neurotrophic Factor; IL—Interleukin; TNF-α—Tumor Necrosis Factor alpha; TLRs—Toll-like receptors; GALTt—Gut-associated Lymphoid Tissue; SCFAs—Short Chain Fatty Acids. This evidence concerns the gene BDNF and autism spectrum disorder.