WST-1 proliferation assay of BT474 cells and SKBR3 cells treated with various retinoid combinations. WST-1 proliferation assay of (a) BT474 cells or (b) SKBR3 cells either untreated or treated for 6 days with vehicle (ethanol + PBS), trastuzumab (Tzmab), various retinoids, or their combinations. Results from WST-1 assays are expressed as a percentage of untreated cells. Each value is the mean of three independent experiments (with eight replicate wells for each treatment) ± standard error. atRA, all-trans retinoic acid; 4-HPR, N-(4-hydroxyphenyl) retinamide (fenretinide); RA, retinoic acid.