ESR1 and breast cancer: WW domain-binding protein 2, encoded by the WBP2 gene, is a breast cancer oncogene.20, 21 WBP2 serves as a molecular on/off switch that controls the crosstalk between ERα,22 WWOX,23 Wnt24 and Hippo signalling networks.25 As a co-activator of ER, WBP2 binds to ERα directly and activates proliferation-related target genes expression to promote the pathogenesis and progression of breast cancer.24 As described above, ERα is critical for chemotherapy resistance in breast cancer.