De novo apparently balanced complex chromosome rearrangement (CCR) involving chromosomes 4, 18, and 21 in a girl with mental retardation: report and review.

We describe a complex and unique, de novo apparently balanced translocation involving chromosomes 4, 18, and 21 with 4 breakpoints, in a patient who was referred for an evaluation of possible fragile-X syndrome. Fluorescence in situ hybridization (FISH) confirmed the complexity of the rearrangement and showed the derivative 21 to be composed of 3 distinct segments derived from chromosomes 21, 18, and 4. The derivative chromosome 18 had undergone a double translocation, the first such event to be described in constitutional complex chromosomal rearrangements (CCRs) involving chromosome 18. A review of these CCRs suggests the existence of a breakpoint "hot spot" on 18q21.