This review provides a concise overview of the structural and functional features of G9a and its involvement in neuropsychiatric disorders, including neurodevelopmental disorders (NDDs) like autism spectrum disorders (ASD) and Prader-Willi syndrome (PWS), schizophrenia (SZ), epilepsy, anxiety, depression, and Alzheimer's disease (AD). The gene discussed is EHMT2; the disease is Alzheimer disease.