The hormone estrogen stimulates the growth of breast cancers that are estrogen-receptor positive, the most common form of breast cancer.
The drug tamoxifen blocks this estrogen effect and prolongs the lives of, and helps to cure, patients with estrogen-sensitive breast cancer.
About 30 percent of these patients have tumors that are resistant to tamoxifen.
This study shows how these resistant tumors survive and grow, and it identifies an experimental agent that targets these breast cancers.