A striking finding from whole-genome sequences is the amount of information we are missing on gene function. For example, although Drosophila is arguably the best-understood multi-cellular organism (and a proven model system for human diseases), mutations with readily detectable phenotypes have been isolated for only about 15% of the more than 15,000 annotated fly genes.

Our lack of information on the majority of genes (the "phenotype gap") does not indicate that the genes have no functions. Instead, it suggests that we have been unable to either assay their roles experimentally and/or resolve an issue of functional redundancy. In addition, our understanding of many genes for which we have some information is limited by pleiotropy, whereby the earlier function of a gene prevents analysis of functions that occur later in development.

How do we systematically learn more information about all genes? Using transgenic RNAi, it is now possible to disrupt the activity of single genes with a spatial and temporal resolution that is impossible or exceedingly difficult to achieve using classical genetic methods.