Effect of paclitaxel on the level of (A) p21WAF1/CIP1 mRNA and (B) p21WAF1/CIP1 protein in SK-BR-3 and MCF-7 cells. After 0, 12, 24 and 36 h of incubation with paclitaxel (100 nM for SK-BR-3 cells and 300 nM for MCF-7 cells), the level of mRNA was determined using RT-PCR and relevant primers and the level of protein was determined using western blot analysis and relevant antibodies (see “Materials and Methods”). Each column of mRNA data represents the mean of 4 experimental values ± SEM. *P < 0.05, **P < 0.01 when comparing the effect with that at 0 h. Actin levels of protein data were used to confirm equal protein loading. In the case of SK-BR-3 cells, MCF-7 cells after 36 h of incubation with paclitaxel were used as a positive control (PC). The data shown were obtained in one representative experiment of two independent experiments.