Because the p53 protein plays a critical role in protecting cells against cancer, researchers want to understand all aspects of its structure and function. In work reported in this issue (p.948) , researchers have now solved the three-dimensional structure of one previously unvisualized section of the protein—a key regulatory region in association with its regulator, a protein called MDM2. In an accompanying paper (p.1001), they also take a look at p539s interaction with another protein that may be involved in controlling the tumor suppressor9s activity.