Mitotic figures were abundant in tumor sections (Figure 3e), and staining of the S-phase marker Ki67 as well as the mitosis marker phospho-histone H3 (Ser10) was readily detectable in tumors, but not in nevus-like structures of control mice (Figures 3h and i, Supplementary Figures S4C and D), strongly indicating a high in-vivo proliferation potential of N-RAS61K-AR cells. Here, MKI67 is linked to neoplasm.