Fig. 4. DNA degradation and double strand breaks are associated with contagious apoptosis. (A) After the indicated regimes of STS/Z-VAD.fmk treatment, the frequency of comet assay-positive cells showing DNA double stranded breaks, was determined by microscopic observation (mean ± s.d., n=3; #P<0.005) in a minimum of 200 cells for each point. (B). Nuclear DNA from samples analyzed in A was subjected to pulse-field gel electrophoresis. Results are representative of three independent experiments.