ESR1 and neoplasm: Therefore, to obviate any complexities arising from direct action of tamoxifen (used to trigger MycERT2 activation) on WMT mammary via endogenous estrogen receptors, we first converted all MMTV-Wnt–induced tumors to ER-negative status by pretreating tumor-bearing MMTV-Wnt1; R26CLSL-MER; R26mTmG mice with tamoxifen prior to their deployment in serial transplantation studies.