This technique allows specific DNA sequences to be directly localized on chromosomes. The probe DNA is labeled by one of several methods that are described in this chapter. The labeled DNA is then applied onto chromosomes fixed on slides under conditions that allow specific hybridization of the probe to its complementary sequence on a chromosome. Following hybridization, unbound probe is removed by washing and the distribution of the probe visualized by autoradiography, immunofluorescence, or enzyme staining.