ESR1 and breast cancer: To validate the results obtained in our in vitro experiments, we tested the anti-tumour activity of AZD1775, neratinib or everolimus in the presence or absence of Fulv in an in vivo PDX model of metastatic ER + CCND1-driven BC with acquired resistance to palbociclib (HBCx-134 palbo-R31) (Fig. 6).