ESR1 and breast carcinoma: Many variables in tissue processing and testing can affect the level of ER expression seen in breast tumors.1–5 For this reason, all invasive and recurrent breast carcinomas are tested for ER expression by immunohistochemistry (IHC) in accordance with guidelines regarding specimen handling, laboratory testing and interpretation of the results.6 In spite of the broad dynamic range of ER expression in both tumor and normal breast epithelium, ER expression in tumors is reported as a final dichotomized, positive or negative result.