Tax stabilizes p27 but not p21. A, Tax increases the half-life of p27 protein. HeLa/18 × 21-EGFP cells were transduced with a lentiviral vector for Tax, LV-Tax, or control lentivirus LV-puro for 48 h. Cells were then treated with cycloheximide (100 μg/ml) for 0, 1, 2, 4, and 8 hours to inhibit de novo protein synthesis and harvested for p27 immunoblots. The level of p27 at each time point was quantified by the Scion Image software. The level of p27 at time 0 was set as 100%. The percent p27 protein remaining at each time point was calculated accordingly, and then plotted. Linear regression analysis of each data set was carried out using the Prism software package. The P value of each line is as indicated. B, Tax has no effect on p21 protein turn-over. The experiment was carried out and analyzed similarly as in (A) except that the rate of p21 turn-over was examined.