Tax activity was suppressed by wild-type p53 when expression constructs for Tax and wild-type p53 were cotransfected. Jurkat, HuT78, and U937 cells were transiently cotransfected with HTLV-I LTR CAT reporter construct (5 μg) and a Tax expression vector (5 μg), together with increasing amounts of an expression construct encoding human wild-type p53 protein. The control vector pCMV-NeoBam was used to normalize the amount of expression vector in each transfection. Values for CAT activity are expressed as a percentage of levels seen in the absence of p53. Experiments were performed ≥3 times. Where not shown, SEM bars are smaller than the symbol size.