Differentiation of BT474 cells or SKBR3 cells treated with various retinoid combinations, determined by Nile red staining of neutral lipids. (a) BT474 cells or (b) SKBR3 cells were either untreated or treated with vehicle (ethanol + PBS), heregulin β1, trastuzumab (Tzmab), various retinoids, or their combinations. On day 6 following treatment, the cells were collected and stained with the fluorescent dye Nile red. Fluorescence intensities of the cells were analyzed by flow cytometry and expressed relative to the intensity of the untreated cells. Results are mean of three independent experiments ± standard error. atRA, all-trans retinoic acid; 4-HPR, N-(4-hydroxyphenyl) retinamide (fenretinide); RA, retinoic acid.