As a result, Gartner can control experiments to a degree previously impossible, exposing identical tissues to different therapies. He can also study how tissue changes with disease. Gartner suspects that when cells become cancerous, the way they attach to each other morphs. So recently he built breast tissue with two cell types to study how they interact. “If we understand how cells assemble,” Gartner says, “we can understand what it is that lets a tissue break down and become metastatic”—a new understanding of cancer itself.