"Metastasis is what most threatens breast cancer patients, and we have found a way to stop the first part of the process in mice," says Andrew Ewald, Ph.D., assistant professor of cell biology at the Johns Hopkins School of Medicine. Before metastasis occurs, single cells on the edge of a tumor, termed leader cells, form protrusions into the surrounding tissue, like someone dipping a toe in to test the water before deciding to venture farther, Ewald says. If the conditions are right, the leader cells act as guides, with many tumor cells following behind, as they escape the confines of the tumor into the healthy tissue beyond. Full metastasis occurs when the cells succeed in migrating to a new location — the lungs, for example — and set up shop, creating a new tumor…