Fig. 11. Effects of p21 and hemin on the differentiation of K562 cells, assayed by the capacity to oxidize benzidine. The mock-transfected K562 clones M1 and M2 were transfected with control vector or the cDNA for p21, respectively, as described in "Materials and Methods." Forty-eight h after transfection, cells at an initial concentration of 0.2 x 106 cells/ml were incubated in culture medium with or without hemin at different concentrations. After 4 days, cells were subjected to a benzidine oxidation test. The fraction of benzidine-oxidizing cells are shown. Mean values from three separate experiments are shown. {square}, culture medium; , 1 µM hemin; , 5 µM hemin; {blacksquare}, 25 µM hemin; bars, SE.