The presence of DNA strand breaks resulting from the cleavage of nuclear DNA by the apoptosis-associated endonuclease(s) is one of the most characteristic features of apoptotic cells (1 ,2 ). A widely used methodology to detect apoptotic cells thus relies on labeling DNA strand breaksin situeither with fluorochromes (3 ,4 ) or absorption dyes (5 –9 ). The advantage of strand break labeling with fluorochromes is that such cells can be rapidly analyzed by flow cytometry. When cellular DNA content is also measured in these cells, the bivariate analysis of such data provides information about the DNA ploidy and cell-cycle phase specificity of apoptosis (4 ,10 ).
