We investigated the involvement of PKR in the changes in frameshift efficiency observed with a low amount of TAR. To this end, PKR expression was silenced by transfecting a PKR siRNA mix into stable 293T transfectants expressing a dual-luciferase HIV reporter. After 24 h, cells were transfected with pTAR in different amounts and harvested 48 h later. As a negative control, an eGFP siRNA mix targeting GFP was used. In the presence of the eGFP siRNA, the frameshift efficiency increases when TAR is present. However, when PKR expression is silenced, this effect disappears, supporting that it is related to PKR activation ( Figure 5A ). Effective silencing of PKR is achieved under the conditions of the assay as shown in Figure 5B . It can be noted that the response of the cells to the increase in the amount of TAR appears to differ from that in Figure 4 . This is due to a difference in the experimental protocol resulting in a lower ratio of the quantity of transfected pTAR to the number of cells (see 'Materials and Methods' section).