An X-linked mental retardation syndrome belonging to the group of conditions with the association of intellectual deficit with hypotonic facies. Prevalence is unknown but the syndrome was first described in 1988 in three males (a 3-year-old boy and his two maternal uncles) from a family in which two other males had died in infancy/childhood. All affected males had a characteristic facies (bitemporal narrowness, almond-shaped palpebral fissures, depressed nasal bridge, anteverted nares, short and inverted-V-shaped upper lip and macrostomia). The surviving patients also had severe intellectual deficit, short stature, mild obesity, hypogonadism and a low total finger ridge count. The syndrome is caused by missense mutations in the ATRX gene (Xq13.3). Inheritance is X-linked recessive.