CASP3 and infection: However, analyzing the kinetics of apoptotic signaling and cellular membrane permeability by simultaneous real-time monitoring of cleavage of a fluorogenic caspase-3/7 substrate and of cell-impermeable Sytox Green (SG) uptake, respectively, showed that MNV-infected primary macrophages displayed SG uptake due to membrane permeability starting from 12 hours post-infection, while caspase-3/7 activity was detected only at later stages of MNV infection (Fig 3A).