Activation or inhibition of PKR is well-known to affect translation initiation via changes in eIF2 phosphorylation (reviewed in 21). However, it is also known that transformed cells, such as those we used in this study, tolerate a certain degree of endoplasmic reticulum stress leading to a certain level of phosphorylation of eIF2 via PERK, a kinase functionally homologous to PKR (34) . Our experimental conditions do not drastically affect the expression of our reporters, implying that the changes in the translation initiation rate caused by activation or inhibition of PKR are small and that the changes in eIF2 phosphorylation should be modest. Using western blotting, we could not detect significant variations in the phosphorylation level of eIF2 in 293T or Jurkat cells transfected with different quantities of TAR (data not shown). We nevertheless suggest that the effect of PKR on HIV-1 frameshift efficiency results from changes in eIF2 phosphorylation that are too small to be detected in presence of the endogenous signal for phosphorylated eIF2 in these cells. However, we cannot exclude that PKR could also influence HIV-1 frameshift efficiency via another yet undiscovered mechanism.