Familial cryptic (20;21) translocation identified by in situ hybridization technologies.

We report on a familial cryptic (20;21) translocation [(t20;21)] that was initially suspected with the observation of a single chromosome 21 specific signal in an interphase nuclei by in situ hybridization (FISH) study performed on a 34-week gestation amniotic fluid specimen. The genetic amniocentesis was prompted by the presence of fetal anomalies detected by ultrasound. In addition, there was a family history of a maternal uncle with mental retardation and multiple malformations and an apparently normal karyotype. No obvious aberration could be detected in the G-banded karyotype prepared from the amniotic fluid specimen. A FISH study using a chromosome 21 specific long arm probe and chromosome 20 whole chromosome paint, however, showed an unbalanced rearrangement in the fetus [46,XY, der(21)t(20;21)(q13.2;q22.13 or 22.2) mat]. The mother and maternal grandmother were demonstrated to be balanced translocation carriers. These results were confirmed by multicolor karyotyping. This familial aberration was discovered by chance in the interphase FISH analysis. Our experience with this case, however, serves to emphasize the importance of the reevaluation of patients with mental retardation and congenital malformations of unknown cause and prudent use of multicolor karyotyping in the detection of cryptic cytogenetic rearrangements.