PTN and neoplasm: Tumor incidence, as determined by the presence of Ki-67+ cells, was strikingly increased in mice injected with RCAS-PDGFB+RCAS-PTN (66.7%) as compared to mice injected with RCAS-PDGFB+RCAS-ev (38.7%) (Fisher's exact test, P = 0.0467) (Figure 2C).