Doctors may be able to determine which women will be helped by the powerful breast cancer drug tamoxifen.
They've pinpointed a molecular on-off switch that the drug uses to attack breast cancer -- the same switch that prevents it from working in some women.
Researchers say that should eventually help doctors test for resistance to the drug. It's the chief treatment for breast cancers that are driven by estrogen.
Tamoxifen doesn't work in as many as one-third of women who are treated with it.
Authors of the study, which appears in the journal Nature, say a test for resistance is still about five years away.