ESR1 and breast carcinoma: In this regard, Xpert® Breast Cancer STRAT4 mRNA (Cepheid, Sunnyvale, CA, USA) (STRAT4) is a real-time quantitative polymerase chain reaction (RT-qPCR) assay that studies the expression of ESR1, PGR, ERBB2 and MKI67 mRNA, providing a positive or negative result for each biomarker in approximately two hours [13].