Transposable elements are mobile genetic elements that can move within genomes. They are found universally in prokaryotes and eukaryotes. There are two classes - those that directly cut and paste DNA, and retrotransposons that use an RNA intermediate and reverse transcriptase. Barbara McClintock discovered transposons in corn in the 1940s-1950s, showing the Ac autonomous element controls movement of the non-autonomous Ds element, causing color patterns in kernels. Transposable elements can impact genes by inserting into or near them, and have played an important role in genome evolution.