Analogous to the perturbations elicited by dysregulation of DNMT3A, TET2, MeCP2, and the DNA methylome, aberrant HDAC2 expression in the brain impairs neurodevelopment, neurotrophic signaling, and HPA axis function in a manner consistent with observations in ADHD, autism, schizophrenia, and the children of maternal smokers [35–37, 67, 86, 88–98, 106, 110, 111, 126]. This evidence concerns the gene HDAC2 and attention deficit-hyperactivity disorder.