A relevant reduction in HVA was detected in two patients with DHPR deficiency (Patients 110 and 113 in Supplementary Table S1), a 1-month-old female with SCN2A-related developmental and epileptic encephalopathy (Patient 1 in Supplementary Table S1), a 4-year-old male with neuronal ceroid-lipofuscinosis type 2 (Patient 80 in Supplementary Table S1), and four patients without an etiological diagnosis (Patients 35, 78, 81, and 84 in Supplementary Table S1). Here, SCN2A is linked to developmental and epileptic encephalopathy.