INTRODUCTION: Resistance to therapeutic agents is a major factor limiting the effective treatment of many forms of cancer. Genes have been identified whose expression and/or overexpression results in resistance of cells to the cytotoxic effects of chemotherapy. Resistance to hormonal agents, however, has received relatively little attention even though the problem of resistance to hormonally active agents is growing in magnitude with the increasingly widespread use of hormone therapy.
Unlike chemotherapeutic agents, which are usually administered as a limited number of courses over a relatively short period of time, hormonal agents are usually given for long periods. In many cases hormonal treatment is withdrawn only when the patient ceases to respond and the disease progresses. The antioestrogen tamoxifen has long been used as the first-line endocrine therapy for women, when they relapse with advanced breast cancer, to provide lowtoxicity palliation. It has subsequently been used as a primary treatment for
