After the milestone discovery of the first microRNA in 1993, the past five years have seen a phenomenal surge of interest in these short, regulatory RNAs. Given that 2% of all known human genes encode microRNAs, one main goal is to uncover microRNA function. Although it has been more difficult to assign function to microRNAs in animals than it has been in plants, important roles are emerging: in invertebrates, microRNAs control developmental timing, neuronal differentiation, tissue growth and programmed cell death. Functional studies in zebrafish and mice point toward important roles for microRNAs during morphogenesis and organogenesis. Finally, microRNAs might regulate viral infection and human cancer VSports手机版. .