Cryptic familial t(11;18)(q25;q23) incidentally detected by interphase FISH

During a prospective prenatal study of numerical abnormalities of chromosom es 13, 18, 21, X and Y using locus-specific probes, we incidentally found a case with only one signal for chromosome 18 per cell in a chorionic villus sampling (CVS) associated with an otherwise apparently normal G-banded kar yotype. This led us to discover a cryptic t(11;18) segregating in a four-ge neration family. The CVS was performed because of mental retardation in the brother to the father of the fetus. A subtelomeric chromosome 18 probe rev ealed one signal on 18qter and one on 11qter of the father. Thus the father had a balanced reciprocal t(11;18) in spite of the apparently normal G-ban ded karyotype. Using the same probes, we found an unbalanced translocation 46,XX,-18, + der (18)t(11;18)-(q25;q23)pat in the fetus. Further investigat ion of the family showed the translocation in balanced and unbalanced form in four generations in mentally normal and retarded individuals, respective ly. The study emphasizes the need for a follow-up with molecular cytogeneti c techniques in dysmorphic and retarded children.