These mutations are thought to contribute to the pathogenesis of this form of EE, although further studies are needed to fully elucidate the biological functions of the wild-type and mutant CYFIP2. In addition to the CYFIP2 mutation, the patient in Case 5 also had a de novo lysine methyltransferase 2D (KMT2D) mutation. The gene discussed is KMT2D; the disease is ethylmalonic encephalopathy.