Researchers commonly use genetically engineered mice to study cancer, but the animal disease differs slightly from the human one. So researchers have sought to transplant human breast tissue into mice to make a better model. The key, Weinberg says, is to transplant two types of human breast cells into the mice, one of which has been blasted with radiation. The cells grow into human-like breast tissues, complete with milk ducts. Unlike human breasts, however, the mice's growths sit flush to the chest. Humans are unusual in this respect, says Daniel Medina, who studies breast cancer at Baylor College of Medicine at Houston, Texas: "In few other species are breasts pendulous." Today's Word is 'Pendulous' | 
