BACKGROUNDPaclitaxel (Taxol) is known to act mainly in mitosis, interfering with microtubule dynamics, but effects on the other cells cycle phases have been reported also. However, a comparative picture of perturbation and killing in the G(1), S and G(2)M phases after drug treatment is lacking. The approach developed by our group tackles the problem of the complexity of cell cycle effects with the aid of a computer program simulating cell cycle progression and new quantities measuring cell-cycle arrest and death.METHODSThe program generates data that were compared with those given by absolute cell counts and by different flow cytometry techniques, enabling us to follow the fate of G(1) and G(2)M blocked cells either re-entering the cycle or dying, distinguishing cytostatic and cytotoxic effects. Apoptosis was analyzed in order to refine the description of cytotoxic effects.RESULTSWe estimated the number of blocked and dead cells after short-term Taxol treatments in a range of concentrations and post-drug incubation times. G(2)M block was immediately active at low concentrations but was reversible, becoming irreversible only at the highest concentrations. G(1)block became active later, allowing cell cycle progression of cells initially in G(1), but was still active 48 h post-treatment, at intermediate concentrations. S-phase delay was detected after 24 h. The death rate was much higher within G(1)than G(2)M blocked cells.CONCLUSIONSOur analysis unraveled the complexity of cell cycle effects of the drug, and revealed the activity of G(1) checkpoint, hidden by a prompter but less cytotoxic G(2)M block.