In the case of breast cancer, resistance often correlates with the loss of estrogen receptor expression and activation of ER (ERα isoform unless otherwise specified) independent growth pathways, such as PI3K/Akt and mitogen-activated protein kinases signaling cascades, as well as the induction of epithelial-to-mesenchymal transition (EMT) (10–13). Here, ESR1 is linked to breast cancer.