An X-linked mental retardation syndrome characterized by facial dysmorphism (slanted palpebral fissures, narrow face, micrognathia, patulous lower lip, flat or small philtrum, and alternating exotropia and ptosis), short stature, early hypotonia and later hypertonia, prominent upper central incisors, foot deformities (pes planus, metatarsus varus, equinovarus), psychomotor retardation and behavioral problems. Prevalence is unknown. Since its initial description in 1980, the disorder has been described in males from 11 families and in one isolated case. In most cases the syndrome is caused by mutations in the ATRX gene (Xq13.3), however, the causative gene in a large Chinese family has been mapped to a 19.8 Mb interval on Xq25. Transmission is X-linked recessive.