Tamoxifen resistant cells are sensitive to Hhat inhibition. A-D, T47D (A), HCC1428 (B), MCF7 (C), BT474 (D), and TamR (E) cells were cultured for 6 days in the presence of vehicle, 10 μM RU-SKI 43 alone or in combination with indicated concentrations of 4-hydroxytamoxifen (4-OH Tam). Cell numbers were quantified and normalized to vehicle treated cells (100 x (drug/vehicle). Bars represent mean ± SD (n = 3) for all panels. Three independent experiments were performed in duplicate using cells at three different passages. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; ****P ≤ 0.0001; Student’s t test.