Medical knowledge has been significantly expanded by the techniques of molecular genetics. A new technology, the polymerase chain reaction (PCR) (1–6), has produced a quantum leap in the field of molecular genetics. PCR uses anin vitrochemical reaction to amplify impure DNA, either fragmented or intact. A defined DNA fragment can be amplified a millionfold in a relatively brief incubation (a few hours). The ability to amplify minute quantities of crude DNA gives the method extraordinary power and sensitivity, DNA can be amplified from fixed pathologic specimens (7), buccal cells from mouth washes (8), a single human hair (9), and solitary cells (10). Once amplified, large DNA samples can be studied by electrophoresis, Southern and slot blot, and other technique