Detection of apoptosis-associated DNA strand-breaks by in situ terminal deoxynucleotidyl transferase assay (TdT-assay) in WSU-CLL and I83CLL cells. The cells were either untreated (CTRL) or exposed flavopiridol (200 nmol/L, 400 nmol/L) for 24 hours, respectively. The dots represent the distribution of individual cells with respect to their b-dUTP incorporation and DNA content. The degree of b-dUTP complexes correlated with DNA strand-breaks per cell.