ESR1 and breast cancer: Moreover, E2/ERα‐66 and its crosstalk with EGFR signaling pathways induces the upregulation of syndecan‐2, a transmembrane heparan sulfate proteoglycan (HSPG) that is mainly involved in the angiogenesis and the EMT process in ERα+ breast cancer cells (Fig. 4) [2, 44].