Mitochondria are required for p21-IRS–induced apoptosis. (A) U937 cells, but not U937 ρ° cells, are susceptible to actimycin A. (B) U937 ρ° cells are largely resistant to p21-IRS–induced apoptosis. (C) p21-IRS does not hemolyze red blood cells. Medium (○); p21-IRS (▴); 1% Triton X (▪). (D) p21-IRS lyses rabbit reticulocytes but not red blood cells.