Quantification of apoptosis. Cells were incubated for 6 days in the absence of retinoids or in the presence of 10⁻⁷ mol/L atRA, 10⁻⁷ mol/L CD367, 10⁻⁷ mol/L CD2425, or the combination of both 10⁻⁷ mol/L CD367 and 10⁻⁷ mol/L CD2425. Count of apoptotic cells was performed on cytospin preparations after MGG coloration, Hoechst 33258 staining, or TUNEL assay. In each case the percentage of apoptotic cells was determined in duplicate counts of 400 cells. Mean values ±SD from six independent experiments are shown.