FBXW7 and infection: Furthermore, the protein levels of p53 were gradually increased during passaging in Fbxw7fl/fl;p53fl/fl MEFs expressing Fbxw7 and p53 as wild-type MEFs after infection with adenovirus-expressing GFP (Ad-GFP) (Fig. 3B, lanes 5 and 3 versus 1), implying a significant role of p53 in replicative senescence.