The use of recombinant DNA technology to develop specific molecular (usually DNA) probes has provided medical and veterinary diagnostic laboratories with powerful new tools to enhance the diagnosis of infectious diseases, genetic disorders, and malignancies, as well as more sensitive and specific means to accomplish such tasks as tissue typing and paternity testing. They are also useful as aids in forensic medicine (Table 1). Application of DNA probes as diagnostic reagents may allow laboratories to reduce turnaround time, broaden the spectrum of agents detected, identified and/or quantitated, and reduce costs by simplifying certain labor intensive determinations and allowing laboratories to perform other tests on-site rather than sending specimens to reference laboratories.‘-” DNA probes are single-stranded pieces of nucleic acid, labeled with a specific tracer (isotope, enzyme, or chromophore), that will hydrogen bond (hybridize) with complementary single-stranded pieces of DNA (or RNA) under the appropriate conditions of pH, temperature, and ionic strength. The stability of the double-stranded complex (hybrid) depends on the degree of complementarity between the two nucleic acid strands. Under stringent conditions (high pH and temperature, low ionic strength), the two strands of an imperfectly matched hybrid will dissociate and the perfectly matched hybrids will remain intact. The hybridization reaction is the basis for any DNA probe assay and consists of four components (Table 2): the probe; the target (the nucleic acid contained in the sample submitted for testing); the detection method, which is based on the specific label or reporter group employed; and the hybridization format.