FBXW7 and infection: We then measured the cumulative population doubling time of MEFs by serial passaging using a 3T9 protocol and found that Fbxw7fl/fl;p53+/+ MEFs underwent senescence at P4 after Fbxw7 deletion by Ad-Cre and Fbxw7fl/fl;p53+/+ and Fbxw7fl/fl;p53fl/fl MEFs underwent senescence at P6 upon Ad-GFP infection (Fig. 3C), whereas Fbxw7fl/fl;p53fl/fl MEFs were immortalized without senescence upon Fbxw7 and p53 deletion by Ad-Cre (Fig. 3B, lanes 2, 4, and 6; Fig. 3C).