ESR1 and breast cancer: In breast cancer cells cultured in absence of hormone, depletion of ERα brings about a response similar to Epithelial-to-Mesenchymal Transition (EMT) [4–6] by activating mesenchymal genes and growth-sustaining pathways and, in vivo, the loss of ERα is usually accompanied by a more invasive and clinically aggressive phenotype [7, 8].