Cells that expressed the estrogen receptor could bind tamoxifen, and the drug, an estrogen antagonist, shut off estrogen responsiveness, thus choking the cells’ growth. But ER-negative cells lacked the receptor for the drug and thus were insensitive to it. The schema had a satisfying simplicity. For the first time in the history of cancer, a drug, its target, and a cancer cell had been conjoined by a core molecular logic.
