Expanded runs of consecutive mixed (GCN)n repeats in the first and second polyalanine tract of the Aristaless‐related homeobox gene (ARX; Mendelian Inheritance in Man [MIM] 300382) have been identified in male children with a severe form of DEE, termed DEE type 1 (DEE1; MIM 308350), also known as infantile spasm syndrome X‐linked type 1(ISSX1).3, 4. Here, ARX is linked to developmental and epileptic encephalopathy.