sentence1: As p16INK4a blocks the inactivation of pRb by cyclin-dependent kinases, and Arf blocks the inactivation of p53 by Mdm2-mediated proteolysis, both have the capacity to cause cell cycle arrest.
sentence2: By inducing Arf and Ink4a in primary rodent fibroblasts, oncogenic Ras expression leads to growth arrest and premature senescence.