A disorder characterized by an exceptionally high skull and severe mental retardation. In one syndrome (Crouzon) these symptoms are associated with exophthalmos (protruding eyes), small orbital (eye) cavities, and increased intracranial pressure. This condition is believed to be due to a single dominant gene with “varying penetrance” (that is, not all who possess the gene are equally affected), since there are wide variations in the degree of skull deformity and exophthalmos. In the Apert syndrome, acrocephaly is combined with hypertelorism (eyes wide apart) and syndactylia (fingers grown together). In the Grieg syndrome (hypertelorism), the high skull is combined with a greater degree of telorism but without the defect in the fingers. There is evidence that these variations are also due to a dominant gene, at least in some families.