Apoptotic DNA fragmentation in a cell-free system. (A through D) Nuclei from the four cell lines indicated on axis in which DNA was labeled with [¹⁴C]-thymidine were incubated for 30 minutes at 37°C with either etoposide alone (100 μmol/L, □) or triton-soluble extracts from untreated (▨) or etoposide-treated (100 μmol/L for 3 hours, ▩) cell lines (A = HL60, B = U937, C = K562, D = KCL22). Apoptotic DNA fragmentation was measured by filter elution assay. Results shown are the mean ± SD of two different experiments performed in triplicate. (E) Agarose gel electrophoresis of nuclear DNA from indicated cell lines before (lane 1, 3, 5, 7, and 9) and after (lane 2, 4, 6, 8, and 10) a 30-minute incubation in the presence of triton-soluble extracts from etoposide-treated U937 cells (100 μmol/L for 3 hours).