Pharmaceutical giant Merck's purchase last fall of Sirna, a San Francisco biotech company specializing in RNA interference (RNAi), was just the latest indication that the technology is gaining momentum. Researchers are currently trying to use RNAi to do everything from treat flu to permanently remove hair. In RNAi therapy, small pieces of ­double-­stranded RNA shut down the genes whose sequences they match. Below is a sampling of RNAi therapies under investigation.

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