Cell cycle analysis. BT474 cells were either untreated or treated with vehicle (ethanol + PBS), 1 μM all-trans retinoic acid (atRA), 1 μg/ml trastuzumab, 1 μM tamoxifen, 1 μM atRA + 1 μg/ml trastuzumab (Tzmab), 1 μM atRA + 1 μM tamoxifen (Tam), 1 μg/ml trastuzumab (Tzmab) + 1 μM tamoxifen (Tam), or 1 μM atRA + 1 μg/ml trastuzumab (Tzmab) + 1 μM tamoxifen (Tam). On day 2 following treatment, both floating and adherent cells were collected and fixed. The percentages of cells in the G0/G1, S, and G2/M phases of the cell cycle were determined by flow cytometric analyses. Results are mean of three independent experiments ± standard error.