Analysis of HIV Tat transactivation in murine cells upon expression of HA-CycT1 mutants. 3T3 murine cells were co-transfected with the HIV-LTR-Luciferase reporter gene, HIV Tat and each of the HA-CycT1 mutants expressing plasmids. 48 hr. post transfection cells were harvested and their Tat-dependent luciferase activities were measured. Luciferase readings were normalized to Renila expression and data are presented relative to the readings obtained in cells that expressed human HA-CycT1-wild-type – set to 100 (assays were measured relative to cells that expressed HIV LTR-Luc reporter and Tat alone). HA-CycT1 mutants are divided to four groups according to their binding to Cdk9 and Hexim1 (Table 1). Asterisks mark specific HA-CycT1 mutants that were severely impaired in their ability to rescue Tat transactivation. Results are representative of the mean value of triplicate wells; error bars show ± SEM.