Confirmation of a cryptic unbalanced translocation using whole chromosome fluorescence in situ hybridization.

We report on a 7-year-old boy with minor anomalies, growth retardation, and developmental delay with an initial 46,XY,der(18)t(18;?)(q23;?) chromosome constitution. To determine the origin of the additional chromosome segment, several candidate regions were identified including 4q and 18q. Clinical comparison showed more similarities to individuals with partial dup(4q) than to those with a dup(18q). Whole chromosome fluorescence in situ hybridization (FISH) was used to demonstrate the correct origin of the translocated region, clarifying the karyotype as 46,XY,der(18)t(4;18)(q28.2;q22.2), thus generating information of clinical importance. This illustrates the use of whole chromosome FISH to identify chromosome regions that cannot be determined conclusively using standard cytogenetic banding techniques.