As multiple studies have shown association between quantitative measurement of ESR1 and ERBB2 mRNAs (which encode ER and HER2 proteins) and clinical outcome on tamoxifen and trastuzumab [9, 10, 11], the use of mRNA‐based tests may be a more robust, reliable, and standardized method to assess these important breast cancer biomarkers [12]. This evidence concerns the gene ESR1 and breast cancer.