ESR1 and breast cancer: The Xpert® Breast Cancer STRAT4 Assay (STRAT4)* is a cartridge-based, real-time quantitative polymerase chain reaction (RT-qPCR) assay with qualitative cut-off values for ESR1-, PGR -, ERBB2-, and MKi67- mRNA expression normalized to a reference gene (CYFIP1) using formalin-fixed paraffin-embedded (FFPE) invasive breast cancer tissue.