Genetic tests for the disorders caused by disease genes could not make it from the laboratory bench to the doctor's office until researchers developed an easy and inexpensive way to copy specific DNA molecules. That way, the small amount of DNA extracted from a person's blood or tissue sample could be multiplied into the large quantities needed for DNA sequencing.

Once again, scientists doing basic research overcame a stumbling block. In a small California biotechnology company, the Cetus Corporation, a young scientist, Kary Mullis, was employed to generate new ideas instead of doing bench experiments. He recognized that rather than relying on bacteria to duplicate selected DNA in a cloning process, he could use just the enzymes--called DNA polymerases--that bacteria themselves use to copy DNA. He developed a method, called the polymerase chain reaction and abbreviated as PCR, that allows the enzymes to be used to amplify any specific DNA sequence in a test tube.

There was only one catch--the method worked only on single-stranded DNA and the heating that is needed to unzip the two strands of DNA kills the polymerases. Fortunately, researchers several years earlier had isolated bacteria that had the amazing ability to thrive at temperatures near that of boiling water in hot springs. Scientists discovered that the polymerase isolated from these bacteria could survive the high temperatures needed for PCR. By the late 1980s, the PCR technique had spawned a number of practical developments, of which gene testing is only one.