Cell cycle analysis of SKBR3 cells treated with various retinoid combinations. Cell cycle analysis of SKBR3 cells either untreated or treated for 2 days with vehicle (ethanol + PBS), trastuzumab (Tzmab), various retinoids, or their combinations. On day 2 following treatment, both floating and adherent cells were collected and fixed. Percentages of cells in the (a) G0/G1 phase, (b) S phase, and (c) G2/M phase of the cell cycle were determined by flow cytometric analyses. Results are mean of three independent experiments ± standard error. atRA, all-trans retinoic acid; 4-HPR, N-(4-hydroxyphenyl) retinamide (fenretinide); RA, retinoic acid.