In addition, an empty protease-recombinant vector was used as a negative control. Forty-eight hours post-transfection, cells were stained with annexin V (a marker of apoptosis) and 7-amino actinomycin D (7-AAD, an indicator of cellular necrosis). Staining was carried out according to general annexin V and 7-AAD staining procedures (BD Biosciences). The percentages of cells positive for annexin V and 7-AAD were determined by flow cytometry for GFP-positive cells.