NPM-ALK activates the p16INK4a pathway in vivo. (A) SA-β-gal staining of frozen sections from 1.5-month-old mice of the indicated genotypes (representative images [magnification ×400] and percentage of positive cells). (B) Percentage of Ki67-positive cells from thymocyte cell-suspensions of 1.5-month-old NPM-ALK transgenic or control littermates, as evaluated by FACS analysis. Results were obtained from 3 independent experiments, each performed using at least 2 mice of each genotype. (C) Survival curves of NPM-ALK transgenic mice of different genotypes for p16INK4a, as indicated. Median time to death is indicated in the inset. (D) Western blot analyses of p16INK4a, p53, and ARF expression in tumors derived from NPM-ALK transgenic mice of different p16INK4a genotypes, as indicated. Tubulin or vinculin was used as loading control. C+ indicates WT MEFs (positive control); C−, p16INK4a- or Tp53-null MEFs (negative controls).