Marker X-associated mental retardation. A study of 150 retarded males.

One hundred and fifty male patients with mental retardation of unknown origin were studied cytogenetically. Six patients (4%) were marker X-positive. All these index cases were found in a subgroup of patients with no dysmorphic features, significant neurological findings or childhood psychosis. In this subgroup the frequency was 8.8%. One of the index cases had a family history suggestive of X-linked mental retardation, four had retarded 1st degree relatives and one had no family history of mental retardation.