Partial distal 12q trisomy.

Two profoundly mentally retarded sisters with partial trisomy for the distal part of the long arm of chromosome 12 (12q24 leads to 12qter) are described. Their anomaly arose as a segregation product of a balanced t(12q-; 17q+) translocation in the mother. Both patients display a similar craniofacial dysmorphism, but lack gross internal or external malformations.