Fig. 5. p53 response to DNA damage in cells stably expressing Mdm2 mutants. Western blot analysis showing endogenous levels of p53 and p21Waf1/Cip1 in pooled colonies of cells stably expressing Mdm2 mutants {Delta}222–437 and 1–440 before and after a 16-h treatment with 5 nM actinomycin D.