Our exome sequencing effort indicated that probands originally diagnosed as RTT-like patients were, in fact, carriers of well-known pathogenic de novo mutations linked to Dravet Syndrome (SCN1A), myoclonic-atonic epilepsy (SCLC6A1), or early infantile epileptic encephalopathies 24 (HCN1) and 27 (GRIN2B). Here, GRIN2B is linked to early-infantile DEE.