The eosinophils have a rearrangedPDGFRBlocus and are part of the neoplastic clone. We used a two-color interphase FISH assay in an eosinophil-enriched preparation to define the clonality of these cells. Differentially labeled BAC clones located centromeric (red) or telomeric (green) to the PDGFRB breakpoint region in the t(1;5) were used as probes. Schematic representation of chromosome 5, location of breakpoint and probes is shown on the left side. In normal metaphases and interphases (top panel) the red and green signals are juxtaposed (yellow) in both chromosomes 5. In the patient's peripheral blood eosinophils (bottom panel), one pair of signals is juxtaposed (normal chromosome 5), whereas the other signals are split, indicating rearrangement of the PDGFRB locus. The eosinophilic granules were pseudo-colorized in white because of their auto-fluorescence. Original magnification, × 1000.