SummaryOur work, centered on hormone dependency hreast cancer, has therapeutic and fundament al implications in two main domains:1. Characterization of hormonal responsiveness by receptor assays:Among clinical, biochemical and pathological data, estrogen receptors (ER) are the best barkers of responsiveness to hormonal treatments. The quantitative assay is particularly informative, and a model linking ER concentration to the probability of response has been created, nich guides the clinician for the treatment of indual patients. It is also helpful for the Prospective study of new treatments on groups of Patients.2. Use of interactions between hormones and Wotoxic drugsOn a murine model, it was shown that one single pulse of estradiol exerts a transientvmitogenic effect. This manipulation allows, under very stringent conditions, to amplify the an titumioral action of a cycle-active cytotoxic drug.The concept is now under study in a randomized clinical trial in which patients receive chmotherapy preceeded either by estrogenic receruitment or by placebo.