AGC1 deficiency, named “early infantile epileptic encephalopathy 39” (EIEE39; OMIM ID # 612949; DEE39, “Developmental and Epileptic Encephalopathy 39”), is an inborn error of metabolism affecting AGC1-MAS activity and producing symptoms that include neurodevelopmental delay, hypomyelination, refractory epilepsy, and severe hypotonia. Here, SLC25A12 is linked to developmental and epileptic encephalopathy.