Down syndrome with pure partial trisomy 21q22 due to a paternal insertion (4;21) uncovered by uncultured amniotic fluid interphase FISH

Objective: To emphasize the usefulness and reliability of fluorescence in situ hybridization (FISH) on uncultured amniotic fluid cells in the prenatal diagnosis of common chromosomal aneuploidies. Methods: FISH analyses utilizing centromeric, locus-specific or whole chromosome paint DNA probes specific for chromosomes X, Y, 13, 18, 21, and 4 were performed on uncultured amniotic fluid cells or the peripheral blood specimen from the father. Routine chromosome analysis was carried out as well. Results: A prenatal case with partial trisomy 21 due to a paternal cryptic insertion (4;21) was ascertained by a rapid overnight FISH on uncultured amniotic fluid cells. The fetus was delivered at term and had classical features of Down syndrome. Conclusion: Our results stress the importance of FISH on uncultured amniotic fluid cells to supplement routine cytogenetics, especially in cases with abnormal ultrasound findings.