The chromosome banding patterns produced by certain DNA binding fluorescent dyes (primary stain ) can be enhanced or modified by an appropriately chosen DNA binding counterstain (Fig. 1 ). The latter ligand is usually nonfluorescent or may be fluorescent, but not in the wavelength range of the primary stain (reviewed in 1,2). Two such methods have proved indispensable in routine human cytogenetic diagnosis, namely, DA-DAPI banding (3 ,4 ) and chromomycin R-banding (5 ). The mechanisms of fluorescence contrast enhancement on counterstaining have been studied in some detail (1 ,2 ,6 –11 ).