Differentiation of BT474 cells as determined by Nile red staining of neutral lipids. BT474 cells were either untreated or treated with vehicle (ethanol + PBS), 50 ng/ml heregulin β1, 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 6 following treatment, the cells were collected and stained with the fluorescent dye Nile red. The fluorescence intensities of the cells were analyzed by flow cytometry and expressed relative to the intensity of the untreated cells. Results are mean of eight independent experiments ± standard error.