Two-color FISH analysis of gene copy number on interphase nuclei and DNA fibers in a patient without IgV_H mutations. (A) FISH to interphase nuclei with slightly extended DNA fibers. (B) FISH to highly extended DNA fibers. Three BAC clones (Figure 2) were visualized either by FITC (green) (AC026113 and AC012083) or by TexasRed (red) (AC063949). Because of the order and differential overlap of the 3 BAC clones, the expected order of the coloring is green-red-yellow-green. Inspection of a high number of nuclei and extended DNA fibers failed to reveal any additional gene copies, and the order of the BAC clones along the chromosomes, when possible to analyze, was as expected.