A clinical trial has begun testing an experimental drug that targets the nucleolus, a structure in the cell nucleus that's turning out to be more powerful than scientists expected. Researchers long thought that the nucleolus's main role was making RNA for ribosomes, the structures that synthesize proteins. But they've discovered that the nucleolus performs many other functions, including regulating the cell cycle, promoting DNA repair, and controlling p53, a key defense against cancer. Scientists have recently shown that the nucleolus forms around specific DNA sequences and that it behaves as a separate phase of matter from the surrounding material. Researchers are just beginning to investigate the nucleolus's role in illnesses such as heart disease and Parkinson's disease.