Endocrine therapy resistance in estrogen receptor (ER)-positive breast cancer

•Endocrine treatment resistance in estrogen receptor positive breast cancer.•Technological advancements for the discovery of biomarkers of endocrine resistance.•Opportunities and restrictions of biomarkers in the clinical laboratory.

Estrogen receptor (ER)-positive breast cancer represents the majority (∼70%) of all breast malignancies. In this subgroup of breast cancers, endocrine therapies are effective both in the adjuvant and recurrent settings, although resistance remains a major issue. Several high-throughput approaches have been used to elucidate mechanisms of resistance and to derive potential predictive markers or alternative therapies. In this review, we cover the state-of-the-art of endocrine-resistance biomarker discovery with regard to the latest technological developments, and discuss current opportunities and restrictions for their implementation into a clinical setting.