As an alternative means to quantify apoptosis, we compared wild-type and Bid-deficient MEFs for the onset of morphological characteristics of apoptosis following reovirus infection using an acridine orange (AO) staining assay ( Figure 2B ). Infection of wildtype cells resulted in a significant increase in the fraction of apoptotic cells at 48 h post-infection with 40% and 100% of the cells exhibiting apoptotic features at MOIs of 100 and 1000 PFU per cell, respectively. In contrast, Bid-deficient cells infected with T3D at either MOI displayed levels of apoptosis equivalent to mock-infected cells, ,10%. Similar results were obtained following infection with another apoptosis-proficient reovirus strain, T3SA+ (data not shown). These data indicate that Bid is required for apoptosis induction following reovirus infection.