ESR1 and neoplasm: Immunohistochemistry (IHC) for clinically relevant markers showed that the mean proliferation index was 42.1% in MM134 xenografts with Ki67 heterogeneously expressed, generally higher in the invasive and lower in the in situ component (Fig 2E); 61.7% of the tumor cells were ER‐positive whereas PR was not detected (Fig 2E).