When the human genome was sequenced two years ago, researchers held the blueprint of a human being in their hands - a nonsensical string of three billion DNA letters. But the project's promise of curing disease and explaining the workings of the human body would depend in part on a curious phenomenon first observed in microscopic worms and petunias - cells' natural ability to "turn off" genes. Over the past few years, scientists have turned the once-puzzling research result into a powerful tool now poised to bridge the gap between the genome project and the medicine cabinet.