Proteins are supposed to be large—they are macromolecules. But researchers have discovered that cells produce hundreds or thousands of tiny proteins, some of which contain only a few amino acids. Scientists had largely ignored these proteins because they didn't meet the size criterion for genomic analyses. But new studies suggest the proteins are abundant and perform a variety of key jobs in cells. Their roles may include controlling the activity of larger proteins, promoting muscle development, regulating muscle contraction, and managing removal of old or faulty RNA molecules. Small proteins are also common in venom produced by spiders, scorpions, and other poisonous animals. Insecticides that use small molecules are already on the market, and researchers are now exploring other practical uses for small proteins, including labeling brain tumors to improve the precision of surgery.