TGFBR1 and anus neoplasm: Increased staining for Ki67, a hallmark of proliferation, was also observed in 2cKO mouse anal tumors as compared with Tgfbr1/Pten 2cKO perianal skin (P<0.01) and Tgfbr1flox/flox/Ptenflox/flox perianal skin (P<0.01, Figs. 3A and B).