Scientists who famously grew a human ear on the back of a mouse may finally have been trumped. Genetically engineered mice have now been used to grow lumps of human breast.

Robert Weinberg and his team at the Whitehead Institute for Biomedical Research in Boston transplanted human breast tissue into mice to help them study the biological changes that give rise to breast cancer.

To get the breast tissue to grow properly, Weinberg injected a mixture of human breast cells - normal epithelial cells and supporting cells known as fibroblasts - into the mammary glands of the mice. The cells grew into human-like breast tissues, complete with milk ducts. But unlike human breasts, the tissue sat flush to the animals' chests. Humans are unusual in this respect, says Daniel Medina, who studies breast cancer at Baylor College of Medicine in Houston, Texas: "In few other species are breasts pendulous."

The human-breasted mice also developed cancer in much the same way as humans, Weinberg reports in Proceedings of the National Academy of Sciences.

Scientists think breast cancer starts when one epithelial cell gets a mutation in its DNA and starts dividing wildly. When the team modified the human fibroblasts so that they made a protein often over-produced in human breast tumours, the mice developed cancer. This suggests that the transplanted epithelial cells were harbouring mutations that turned cancerous in response to signals from neighbouring cells. Researchers hope that the new system will help them tease out these signals and, perhaps, find ways to stall them. "It's a very big advance," says Medina.