Despite a well-known role of Apaf-1 in the activation of caspase-9 as the initiation of caspase cascade in a variety of cell models, several reports demonstrated that alternative mechanisms for caspase-9 activation exist independently of Apaf-1 on the basis of certain stimuli, such as the infection of Sendai virus in apaf-1−/− MEFs [45] and UV irradiation in apaf-1fog/fog cells [46]. This evidence concerns the gene CASP9 and infection.