Etoposide-induced apoptotic DNA fragmentation and expression of Bcl-2/Bax proteins in four human leukemic cell lines. (A) DNA fragmentation was measured by a filter elution assay in HL60 (•), U937 (▪), K562 (○), and KCL-22 (▵) cell lines treated for 24 hours with indicated concentrations of etoposide (VP-16). (B) DNA fragmentation was identified as internucleosomal by agarose gel electrophoresis. Cells were treated for indicated times with 100 μmol/L etoposide.