Published Jan 1, 2007 8:00 AM

04. Get a Clue

What: DNA fingerprinting

Who: Biochemist Kary B. Mullis, working for Emeryville-based Cetus Corporation

Impact: Never heard of polymerase chain reactions? Not surprising. But you almost certainly know about CSI or Law & Order. And almost everybody recalls when blood and hair samples were introduced by prosecutors during the O.J. Simpson murder trial (in which Mullis testified for the defense). Cops and prosecutors use a technique called DNA fingerprinting — duplicating a certain section of DNA and matching it to another — to get the bad guys and to prove paternity. That's polymerase chain reaction (PCR), which matches the rungs of a ladder-shaped DNA molecule with a polymerase enzyme and artificially gets the ladder to copy itself. PCR has allowed scientists to detect viruses and genetic diseases, clone genes, map the human genome and pay the bills of countless law firms.

Eureka moment: Mullis was driving up California Highway 128 for a weekend in Mendocino, mulling techniques to analyze small mutations in DNA, when the inspiration that led to PCR hit. Mullis says he pulled over, exclaimed "Dear Thor!," and spent the next two days writing equations and drinking wine.

— Eric Vance