The most common methods utilize HIV-1 isolates or recombinant viruses and some of them are commercially available. However, phenotyping has three basic limitations: the high biological risk; the complexity of the molecular basis; and the high cost. Lindsten et al. and Majerova-Uhlikova et al. reported a new protease assay in eukaryotic cells based on the expression of an HIV-1 protease fused to the green fluorescent protein (GFP). The fusion protein (GFP–PR) is activated in vivo by the autocatalytic cleavage of HIV-1 protease, resulting in the rapid elimination of protease-expressing cells, due to the cytotoxic effect of the protease. Treatment with protease inhibitors results in a dose-dependent accumulation of the fluorescent precursor, which can be detected and quantified