We describe two patients, who were assessed at a child psychiatry clinic, with a known genetic disorder in the arhgef9 gene on the long arm of the X chromosome. The boys presented with developmental delay, hyperactivity, autism spectrum disorder and epilepsy. This prompted us to conduct a literature search on previously identified patients with the same mutation and its impact on psychiatric symptoms in children. Recent evolutions in genetic technologies have led to the possibility of identifying and investigating more children with developmental disorders with or without psychiatric disorders. Child psychiatrists are confronted with the question what role genetics play in the child’s clinical presentation. A basic knowledge of genetic principles is now required.