Despite an avalanche of RNAi research over the past decade, however, a nagging question remained mostly unanswered: what good is RNAi to the organism itself?Substantial roles for RNAi in regulating endogenous gene expression have been difficult to ascertain because Drosophila melanogaster1,2 and Caenorhabditis elegans3,4 mutants that selectively inactivate RNAi seem to be normal and fertile.