We describe several genes-including <i>CDKL5, TSC1/2, SCN1a,</i> and <i>TANC2</i>-that have been associated with epilepsy, ASD, or other NDD phenotypes that play a critical role in regulating one or more stages of brain development or function but differ widely in their disease-causing mechanisms. The gene discussed is CDKL5; the disease is Neurodevelopmental delay.