It has long been known that about one-fifth of human breast cancers are hormone-dependent in that removal of the estrogen-producing organs or, more recently, administration of medication that prevents either the synthesis or action of estrogen leads to a remission of the malignancy.There was need for a means of predicting which cancers will respond to anti-estrogen therapy so that the majority of patients can be placed directly on chemotherapy and spared a treatment that will not help them.The demonstration that estrogens exert their action in combination with a specific receptor protein suggested that breast cancers with low or negligible receptor content have lost hormone dependency whereas those responsive to anti-estrogen therapies would contain the receptor on which hormone action depends.