Cerebral folate deficiency was demonstrated in nine patients (one with KCNH1-related developmental and epileptic encephalopathy, one with DHPR deficiency, one with Glut1 deficiency, one with a PRRT2 pathogenic variant, and five with no established etiological diagnosis). The gene discussed is KCNH1; the disease is hyperinsulinemic hypoglycemia, familial, 4.