In this study, we investigated whether the presence of TAR affects HIV-1 frameshift efficiency in relationship with the changes it causes in the rate of cap-dependent translation initiation. To this end, we used a dualluciferase construct (8) which expresses the Renilla luciferase (Rluc) and the firefly luciferase (Fluc) separated by HIV-1 frameshift region as a fusion protein. Rluc is expressed following conventional rules of translation whereas Fluc expression requires a -1 frameshift in the HIV-1 frameshift region. This type of construct is adapted from Grentzmann et al. (22) , who pioneered the use of a dual-luciferase reporter for studying recoding signals. CD4+ T cells (Jurkat) or 293T cells were transfected with the dual-luciferase plasmid and TAR was added either in cis or in trans of the reporter mRNA. Several conditions were assayed to characterize the effect of TAR on frameshift efficiency and the involvement of PKR in this effect, such as the introduction of a small or a large amount of TAR in the cells, the use of mutants of TAR that cannot perturb PKR activity and the silencing of PKR expression with short interfering RNA (siRNA).