When doctors diagnose and plan treatment for breast cancers they look for various indicators of how aggressive they are and what treatments will work best. Two-thirds of breast tumors are made up of cells that have receptor molecules for estrogen on their cell surface. The growth of these estrogen-receptor-positive (ER+) tumors depends in part on estrogen in the woman’s body. Treatment for these cancers, along with surgery, usually involves a hormone blocker such as tamoxifen. But there’s no such method to treat ER- tumors, which are independent of estrogen. So researchers at Dana-Farber Cancer Institute have been searching for other ways to stem their uncontrolled growth. Debajit K. Biswas, research associate at Harvard Medical School and senior associate in the laboratory of Arthur B. Pardee, has used a drug known as Go6976 to treat ER- breast cancer tumors in mice. “This drug is inhibiting the growth of tumors and causing fully-grown tumors to regress and go away, and the mice show no signs of toxicity,” says Biswas. The scientists note this was a small study involving 20 mice, and there are no current plans to try it in people. “It’s still a big jump to the human,” cautions Pardee.