Wild-type, Bid-deficient, or TRAIL-R-deficient MEFs (5610 4 ) were grown in 24-well plates (Costar) and adsorbed with reovirus at room temperature for 1 h. The percentage of apoptotic cells after 48 h incubation was determined using AO staining as described [6] . For each experiment, .200 cells were counted, and the percentage of cells exhibiting condensed chromatin was determined by epi-illumination fluorescence microscopy using a fluorescein filter set (Zeiss Photomicroscope III; Thornwood, NY).