TP53 and neoplasm: Given the previously demonstrated ability of ARF to induce cell cycle arrest and apoptosis independently of p53 [9], [18], [19], [20], [21], we examined for changes in apoptosis and proliferation as assessed by TUNEL labeling and BrdU incorporation, respectively, in pre-neoplastic and tumor lesions from RIP-Tag2; Arf+/+ and Arf−/− mice.