“…The Ha-ras val12 oncogene, for example, induces growth arrest and senescence in primary cell cultures (Serrano et al, 1997), although in established cell lines it induces oncogenesis (Marshall, 1996;Lloyd, 1998;Hanahan and Weinberg, 2000). In primary cultures, Ras evokes growth arrest by activating proteins such as p53, p21, p16INK4A, CDK4 and p19ARF (Serrano et al, 1997;Ries et al, 2000;Lazarov et al, 2002). Development of an oncogenic phenotype in primary cultures may occur upon inactivation of one of them (Serrano et al, 1997;Sahai et al, 2001).…”