Pericentromeric euchromatin is conserved in minute human supernumerary chromosomes: a study using cross-species colour segmenting (RxFISH).

Investigation of marker chromosomes is one of the most challenging areas of clinical cytogenetics, especially in the prenatal scenario. A range of techniques including microdissection/reverse painting, SKY and M-FISH are available for the investigation of larger markers (>3 Mb). All these techniques rely on hybridization of unique, homologous sequences with simultaneous suppression of repeat sequences. In contrast, RxFISH is based on hybridization of cross-species syntenic sequences; repeat sequences do not hybridize due to species divergence. We have used RxFISH to analyse a group of the smallest, i.e. minute, supernumerary marker chromosomes. Our results suggest that even the smallest marker chromosomes often contain conserved pericentric euchromatin. More detailed characterization of pericentric genetic content is needed to assess the clinical significance of minute supernumerary markers.