MicroRNA (micro RNA, miRNA)
MicroRNA (or micro RNA, or miRNA) are segments of RNA that are transcribed from DNA in a way similar to messenger RNA. Some are found within introns (segments of mRNA that are removed after transcription, before translation). The key difference is their length - much shorter at only 21-23 nucleotides long, as opposed to about 2,000 for a typical mRNA - and they are not translated into proteins. Instead, they provide a mirror image match to portions of one or more mRNA molecules (that is, are complementary to them), with which they can pair-bind, thereby shutting down polypeptide translation and blocking production of the protein. In many cases, pair-binding also marks the mRNA for destruction.
In short, instead of producing a protein to oppose its matching mRNA's protein, microRNA blocks mRNA directly, which removes a layer of complexity and allows for more precise timing in controlling gene expression.
The first microRNA was discovered in 1993, but their full significance wasn't apparent until 2000, when a second one was found to be widespread throughout the animal kingdom. Thousands are now known, and they are involved in regulating gene expression in all aspects of running a living organism. Proteins still turn genes on and off, but microRNAs control how many proteins are made. Turning on a gene for one protein might also turn on a gene for a microRNA that stops production of a second protein. Moreover, many diseases have distinct patterns of microRNA gene expression. This makes them excellent for diagnosis, for future development of drugs that target them, and maybe even as future drugs themselves.