p21-IRS–induced apoptosis in U937 cells. U937 cells were incubated with indicated amounts of p21-IRS for 3 hours (A) or with 20 μM p21-IRS for the indicated times (B). Each point was the average of 3 independent experiments. (C) Internucleosomal DNA fragmentation. (D) Release of AIF and cytochrome C from mitochondria in cells treated with p21-IRS. M indicates mitochondria; C, cytosol; cyto C, cytochrome C.