Estrogen Receptor: Methodology Matters

The estrogen receptor (ER) has been our most important biomarker in breast cancer for more than 30 years, largely because of its role in indicating the potential of patients to benefit from endocrine therapy. The substantial survival benefit from tamoxifeninER-positivebutnotER-negativetumors, 1 andtheextension of this benefit with aromatase inhibitors in postmenopausal patients, has made treatment with one of these well-tolerated endocrine agents mandatory in ER-positive disease; therefore, ER measurement has been rendered essential for all invasive primary breast carcinomas. ER-positive tumors have a better prognosis in the early years after diagnosis, which is at least partly due to their slower proliferation, although it has become clear that recurrencefree survival curves come together after about 15 years, 1 which