ESR1 and breast cancer: Indeed, in BC cell lines, in which overall HER2 mRNA was found to reflect tumor addiction to HER2, pHER2 levels, activation of HER downstream signals (mainly AKT) and response to anti-HER2 drugs, we found an increase in HER2 transcription in cells grown in the absence of estrogens or in which ER degradation is induced by fulvestrant.