Apoptosis in K562 cells treated continuously with 68 μmol/L etoposide. (A and B) Morphology of K562 cells after 24 hours (A) and 48 hours (B) continuous exposure to etoposide. Arrow in (A), peripheral chromatin condensation in K562 cell after 24-hour etoposide exposure. Bar, 2 μm. (C and D) HL-60 cells treated with etoposide for 0 to 6 hours (lanes 1 through 6) and K562 cells treated with etoposide for 0 to 48 hours (lanes 7 through 10) were harvested for agarose gel electrophoresis (C) and SDS-PAGE followed by blotting with C-2-10 anti-PARP or polyclonal anti-lamin B₁ antibodies (D). Arrow indicates 89-kD cleavage product of PARP.