In situ hybridization may be defined as the detection of nucleic acids in situ in cells, tissues, chromosomes, and isolated cell organelles. The technique was described in 1969 by two separate groups, who demonstrated repetitive ribosomal sequences in nuclei of Xenopus oocytes using radiolabeled probes (1,2). Refinements in recombinant DNA technology and the development of nonisotopic probe labeling and detection obviate the need for radiation protection and disposal facilities, and have converted nonisotopic in situ hybridization (NISH) from a purely research technique to one that can be used in routine laboratory testing.