While tumor suppressive role of p21, a bona fide p53 target gene, is primarily attributed to its canonical function in inducing cellular senescence and cell cycle arrest as “universal inhibitor” of cyclin kinases, many recent studies demonstrated oncogenic potential of cytosolic p21 through inhibition of apoptosis and activation of cyclin D–CDK4/6 complexes [39, 56–58]. Here, CDK4 is linked to neoplasm.