ESR2 and cholangiocarcinoma: In CCA cells, expression of estrogen receptor (ER)-α and ER-β, whose activation modulates the balance between proliferation (mediated by ER-α) and apoptosis (mediated by ER-β) [26], can affect response to both tamoxifen, an ER antagonist [27], and KB9520, a selective ER-β agonist [28].