Contrast-enhanced, look-up-table (LUT)-inverted digital images of DAPI-banded chromosomes after fluorescence in situ hybridization (FISH) permit direct regional chromosomal localization of the fluorescent signals of single-copy gene probes. Improved quality and resolution of chromosome banding allowed a refined localization of the erbB-3 protooncogene from chromosome band 12q13 to sub-bands 12q13.2-13.3. This procedure can be used for direct and precise mapping of single-copy genes on both normal and cancer-cell-rearranged chromosomes.