Multicellularity has its advantages, but they come at a price. The division of labor in a complex organism means that every cell must perform its job and only its job, so an elaborate regulatory system evolved to keep cells in line. Nearly every one of the trillion or so cells in the human body, for instance, contains a full copy of the genome – the complete instruction set for building and maintaining a human being. Tight controls on which genes are activated, and when, inside any given cell determine that cell’s behavior and identity. A healthy skin cell executes only the genetic commands needed to fulfill its role in the skin. It respects neighbor-ing cells’ boundaries and cues, and when the regulatory systempermits, it divides to generate just enough new cells to repair a wound, never more.