CDKN2A and neoplasm: The evidence that ARF is a critical tumorsuppressor in mammals (Sherr, 2006, Sharpless, 2005), but opposes regenerationfunctions (Sharpless and DePinho, 2007),suggests that the selective pressure that has driven the evolution ofARF has primarily enhanced tumor suppression either at the expenseof or in the absence of regeneration pressures.