In apoptotic execution, cells perform final processes, such as DNA cleavage and nuclear condensation before engulfment by other cells. The molecular mechanisms for nuclear condensation are still unclear, while upstream cytoplasmic pathways are well characterized.Earnshaw et al. have developed a cell-free system which allows isolated nuclei could undergo apoptosis in vitro as intact cells did. We have previously proposed that nuclear condensation could be classified into three distinct steps, stage 1 ring, stage 2 necklace and stage 3 nuclear collapse. Time-lapse imaging of individual nuclei showed all nuclei could follow a reproducible program of condensation, suggesting the existence of an ordered biochemical pathway. We found the necessity of caspase-6 activity for stage 3 nuclear collapse. In conclusion, kinase activity in stage 1, DNase activity in stage 2, and caspase-6 activity in stage 3, are essential for each condensation stages.