Functional activities of Tax and Rex expressed from H1IT. (A) Transiently transfected 293T cells were assayed for Tax activity by cotransfection of wt HTLV-1, H1IT, or an empty vector along with the Tax reporter LTR-1-luciferase. CMV-βgal was used to control for transfection efficiency. The data shown is representative of three independent experiments. There was no significant difference in Tax activity (20- to 30-fold induction over background) when it was expressed from either proviral clone. (B) Transiently transfected 293T cells were assayed for Rex activity by cotransfection of wt HTLV-1, H1IT, or an empty vector along with pcTat and pCgag-RxRE reporter. CMV-Luc was used to control for transfection efficiency, and data shown are representative of three independent experiments. Rex activity measured as a function of p24 Gag production using ELISA was not significantly different between the two proviral clones.