Scientists have recently recognized an apparently fundamental cellular mechanism that may be a sort of ancient immune system. Called RNAi, for RNA interference, the process utilizes pieces of double-stranded RNA to prevent gene expression. RNAi has quickly become a powerful research tool, but its real promise lies in its potential to generate a brand-new class of highly specific medicines. Researchers have only just begun to explore how RNAi works in mammalian cells, so there's no telling if it actually will give rise to therapeutics. A growing number of companies are betting they'll be able to turn the fad into fortunes. Already, some of the challenges to commercialization are obvious: how to deliver the RNA, and who if anyone will control intellectual property rights. Some firms aim to directly deliver short sequences of RNA, and those who travel this route may confront challenges similar to those faced by antisense companies. Others are trying gene therapy approaches and will inherit all the problems of that field. As in any new field, bluster and blather are mixed with secrecy about who has what in terms of technology, money, and patents. The winner is likely to be the first company that can reduce concept to practice, and come up with a drug that works.