ESR1 and breast carcinoma: An in vitro CTC model system focusing on clinically useful treatment predictive biomarkers in breast cancer, specifically the estrogen receptor α (ERα) and the human epidermal growth factor receptor 2 (HER2), was established using healthy donor blood spiked with breast cancer cell lines MCF7 (ERα+/HER2−) and SKBr3 (ERα−/HER2+).