CDKN2A and neoplasm: In addition, while the appearance of solid tumors was very rare at 8 weeks in RIP-Tag2; Arf+/+ mice (tumors present in 4/11 mice; average of <1 tumor/mouse), small tumors were found in nearly all RIP-Tag2; Arf−/− mice analyzed at this age (average of ∼2.5 tumors/mouse; Figure 4C, p<0.0001).