Specific fluorescent bands on chromosomes produced by acridine orange after prestaining with base specific non-fluorescent DNA ligands.

Metaphase chromosomes stained with acridine orange exhibit uniform yellow-green fluorescence. Chromosome preparations treated with the non-fluorescent A-T specific antibiotic distamycinA prior to acridine orange staining exhibit longitudinal fluorescent banding patterns similar to those produced by a number of fluorescent R-band techniques. Similarly, chromosome preparations treated with the non-fluorescent G-C specific antibiotic actinomycin D followed by acridine orange staining exhibit "Hoechst-type" banding patterns. Interactions of various ligand-DNA combinations in solution indicate that the base pair specific antibiotics induce banding patterns by selectively altering acridine orange binding sites in chromosomal regions rich in the particular base pair for which the antibiotic exhibits specificity.