The era of modern molecular biology began in the 1950s when James Watson and Francis Crick discovered the double helical structure of deoxyribonucleic acid (DNA). This discovery established the “central dogma” of molecular biology. Genetic information is codified in DNA. This information is copied to a messenger ribonucleic acid (mRNA). The genetic code in the mRNA is then used to generate a protein. Recent work by many individuals has culminated in the sequence determination of the entire human genome. The genome sequence has been immensely valuable for the identification of genes that contribute to disease, but it also has raised new questions. For example, most of the DNA in the genome does not seem to code for proteins. These regions were sometimes termed “junk DNA” because they did not have an obvious purpose. It is now known that important genes are hidden in these uncharacterized regions of DNA. These include genes that do not code for protein but instead generate biologically active RNAs. One very important type of these “noncoding RNAs” is the microRNA.