The culprit behind mad cow disease, a.k.a. bovine spongiform encephalopathy, is the most infamous mammalian form of prions. Prions are misfolded proteins that are capable of growing, replicating, and being passed on to daughter cells, that is, they are by themselves heritable. Beyond their disease manifestation, prions also occur naturally in some organisms (such as yeast) and may play important roles in their growth and development. Now, Osherovich and colleagues have identified the amino acid sequences that allow prions to aggregate and replicate, and thereby pass through generations of cells - and prove this by designing an artificial yeast prion that does not exist in nature.