The first chromosome banding technique was developed by T. Caspersson and coworkers who used alkylating fluorochromes for the differential staining of Vicia faba chromosomes. It was found that specific chromosome regions show a bright fluorescence after staining with quinacrine (Q) or quinacrine mustard (QM). Whereas in mammalia the Q-banding patterns generally correspond to those obtained by G-banding, in plants Q-bands usually correspond to regions of constitutive heterochromatin that stain dark after Giemsa C-banding. By using AT- and GC-specific fluorochromes and counterstaining with base-specific nonfluorescent drugs, D. Schweizer developed a method of reverse fluorescent banding. The N-banding technique was originally developed for differential staining of the nucleolus-organizing regions (NORs) in animal and plant chromosomes. The Giemsa C-banding technique was discovered by chance and originated from an experiment where tritium-labeled mouse satellite DNA was hybridized in situ to mouse chromosomes.