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The flatworms known as planaria are proverbial for their ability to regenerate a head or tail properly when amputated (first clip in movie). But how does the organism "know" which end to regenerate? Gurley et al. found an answer in a molecular "switch", beta-catenin, a protein that regulates a variety of cell processes during development. By tweaking parts of the beta-catenin system in the freshwater planarian Schmidtea mediterranea using RNA interference, the team was able to specify regeneration of two-headed worms from trunk fragments (second clip), and to create two-tailed worms (third clip). The same protein, the team proposed, could play a similar switching role in the adult tissues of other animals.