This syndrome is characterised by myoclonic epilepsy with generalised spasticity and intellectual deficit. It has been described in six males from two generations of one family. Transmission appears to be X-linked recessive and the syndrome is caused by mutations in the aristaless-related homeobox gene (<I>ARX</I>, Xp22.13).

Subnormal intellectual functioning which originates during the developmental period. Intellectual disability, previously referred to as mental retardation, has been defined as an IQ score below 70.