A possible mechanism proposed to explain how HIV-1 protease could decrease GFP expression,
after the transfection of plasmids containing the GFP–PR, was through cell death induced by HIV-1 protease.In order to
ascertain the molecular mechanisms involved in phenotyping, cell viability assays were performed with annexin V (apoptotic marker) and 7-AAD (necrotic marker). 293T cells were transfected with pcDNA3_GFP_PR-wt at saquinavir concentrations ranging from 0 to 2 mM. The results show that the percentages of annexin V-positive cells in all cases (including negative controls) were equal, independent of the drug concentration (Figure 1a). The same results were observed after staining with 7-AAD (Figure 1a). Consequently, the loss of GFP signal when the protease is fully acting on the system (in the absence of drug) is not due to cell death.