ERβ improves sensitivity to tamoxifen in breast cancer cells. T47-DERβ cells and MCF-7ERβ cells were treated for 5 and 7 days, respectively, with 10 ng/mL (-ERβ) or 0.01 ng/mL (+ERβ) doxycycline in the presence or absence of 10 nM 7-bromo-2-(4-hydroxyphenyl)-1,3-benzoxazol-5-ol (WAY) or 10 nM 17β-estradiol (E2). Thereafter cells were exposed to 1,000 nM 4-OH-T. At indicated time points, cell numbers were measured using a WST-1 colorimetric assay as described in Materials and methods. Significant differences were analyzed using one-way ANOVA and Tukey's multiple posttest. *P < 0.05 vs. -ERβ. **P < 0.05 vs. +ERβ.