CASP9 and infection: Upon PRRSV infection, the cleaved caspase-9 was undetectable until 24 h pi, while it became evident at 36 h pi and 48 h pi in comparison with the uninfected cells, and their significance were analyzed by densitometry analysis, suggesting that PRRSV-induced cleavage of caspase-9 is involveed in intrinsic pathway of apoptosis during the late infection.