Balanced translocations involving chromosome 12: report of a case and possible evidence for position effect.

A girl with mental retardation, a convulsive disorder and clinical findings resembling cerebral palsy had an apparently balanced 12/18 translocation. Three additional cases reported in the literature had an abnormal phenotype associated with the positioning of segments from other autosomes adjacent to the band 12q21. Penotype abnormalities were not present in 14 individuals in which the translocation involved breaks at other sites on chromosome no. 12. These findings suggest the possibility of a position effect on chromosome segments translocated adjacent to the region 12q21.