To identify which if any of these two proteins is a critical downstream effector of Ras that mediates the cytarabine-induced loss of clonogenicity, we generated Ras cells that express either Cdk4R24C, a melanoma-derived mutant of Cdk4 that is resistant to inhibition by p16Ink4a[37] or a dominant-negative allele of p53 [38]. Here, TP53 is linked to melanoma.