Small metacentric nonsatellited extra chromosome: report of five mentally retarded individuals and review of literature. Contribution to further delineation of a new syndrome.

Five mentally retarded male patients with a supernumerary small metacentric nonsatellited chromosome were found to have many clinical features in common. The face showed characteristic small crowded features, the bodily habitus was asthenic, and the hands and feet had minor abnormalities. Renal anomalies were present in two patients. One patient had a myelomeningocele. Cytogenetic studies employing Q, R, and C banding in four patients showed the small extra chromosome to have staining properties compatible with an isochromosome of the short arm of chromosome 18. A comparison with previous case reports suggests a new syndrome. However, the identity of the extra chromosome has not yet been determined.