Recent discoveries related to microRNAs, RNA interference, small interfering RNAs (siRNAs), and small modulatory RNAs have revealed a new class of mechanisms of gene regulation that are mediated by small, noncoding RNAs. Among these small RNAs are the microRNAs. These are thought to control gene expression at the post-transcriptional level by degrading or repressing target messenger RNAs (mRNAs). Surprisingly simple and elegant, microRNA-mediated gene regulation is guided by the base-pairing rules of Watson and Crick. MicroRNAs are individually encoded by their own set of genes and are an integral component of the genetic program. Some are located in noncoding regions of the genome, whereas others occur in the introns (noncoding regions) of protein-coding genes. Furthermore, as shown in the figure, a complex set of proteins is required for the formation and function of microRNAs. Many of these proteins, as well as the microRNAs themselves, are found in a wide range of animal species and are important for development.

There is accumulating evidence that microRNA-mediated gene regulation has a broad impact on gene expression. First, microRNA genes constitute about 1 to 5 percent of the predicted genes in worms, mice, and humans—there may be as many as 1000 microRNA genes in the human genome. Second, microRNAs are expressed at high levels in animal