ESR1 and neoplasm: Studies conducted using a nude mouse model of BT-474 cell-derived xenografts grown in an estrogen-rich environment demonstrated that RO suppressed tumor growth in vivo. Immunohistochemical analysis of tumors collected at the end of the study showed that levels of ERα were also reduced in vivo, though in contrast to our observations using cultured BT-474 cells, up-regulation of ERβ was not as obvious.