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Photo Illustration: Holly Lindem 
In the late 1980s, a molecular geneticist named Richard Jorgensen and a colleague were trying to create a petunia that was a deeper shade of purple, when something went wrong: "Instead of more color, we got white flowers. It was completely unexpected," he recalls.

It took researchers 10 years to figure out what Jorgensen, now a professor in the department of plant sciences at the University of Arizona, had done: He caused a happy accident that has led to a major medical breakthrough, allowing scientists to close in on effective therapies—perhaps even cures—for cancer, HIV, diabetes, Alzheimer's, and other serious diseases.

Jorgensen had synthesized pigment genes, which he injected into the petunias. But when he introduced copies of the gene to the flowers, he unknowingly set off a viral defense mechanism scientists now call RNA interference, or RNAi. The remarkable thing was that RNAi not only shut down the new pigment genes, it deactivated all the pigment genes in the flower.

Once scientists figured out what had happened (two of them won the Nobel Prize in 2006), they realized they might be able to turn off genes that were disease related. If they could inject specially designed RNA into, say, a cancer tumor, RNAi might wipe out the tumor.

And that's exactly what happened—at least in labs. Researchers have stopped ovarian cancer cells and HIV from replicating. "This is a sea change," says Gary Ruvkun, PhD, a professor of genetics at Harvard Medical School who last year was cowinner of a Lasker Award for his RNA discoveries. (Conversely, the RNAi off-switch discovery has led to another stunning breakthrough: a genetic on switch known as RNA activation, or RNAa. Being able to turn genes on and keep them on may help protect bone, muscle, and nerves from the decline that accompanies aging.)

Numerous pharmaceutical companies are already developing RNAi drugs for cancer, HIV, and Alzheimer's, among other diseases. One company that seems close to bringing a treatment to market is Opko Health. Scientists there are currently enrolling participants for phase III clinical trials for a drug designed to treat wet age-related macular degeneration, a common cause of blindness. These RNAi-based drugs could be available within the next five years.

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