New BAC probe set to narrow down chromosomal breakpoints in small and large derivative chromosomes, especially suited for mosaic conditions.

Fluorescence in situ hybridization (FISH) and/or array-comparative genomic hybridization (aCGH) performed after initial banding cytogenetics is still the gold standard for detection of chromosomal rearrangements. Although aCGH provides a higher resolution, FISH has two main advantages over the array-based approaches: (1) it can be applied to characterize balanced as well as unbalanced rearrangements, whereas aCGH is restricted to unbalanced ones, and (2) chromosomal aberrations present in low level or complex mosaics can be characterized by FISH without any problems, while aCGH requires presence of over 50 % of aberrant cells in the sample for detection. Recently, a new FISH-based probe set was presented: the so-called pericentric-ladder-FISH (PCL-FISH) that enables characterization of chromosomal breakpoints especially in mosaic small supernumerary marker chromosomes (sSMC). It can also be applied on large inborn or acquired derivative chromosomes. The main feature of this set is that the probes are applied in a chromosome-specific manner and they align along the chromosome in average intervals of ten megabasepairs. Hence PCL-FISH provides denser coverage and a more precise anchorage on the human DNA-sequence than most other FISH-banding approaches.