Immunostaining of paraffin-embedded sections confirmed the Western analysis, with nuclear staining of p53 detected in the epidermal cells of papillomas from wild-type mice, reduced numbers of p53-positive cells in the p19 Arf heterozygous papillomas, and undetectable p53 staining in p19 Arf-null papillomas (Figure 3B). Here, CDKN2A is linked to papilloma.