“…Regardless of the stoichiometry and PCNA binding arguments, and based on the previously mentioned finding that p21/cyclin/CDKs can exist in active complexes, more recent work in our and other laboratories has led to the discovery that p21 is required for G1→S phase progression in at least some cell types. The role for p21 as an assembly factor for cyclin D1/CDK4 complexes (Weiss et al, 2000;LaBaer et al, 1997) helps explain the heretofore mysterious result that p21 levels are increased soon after mitogen stimulation. Data is emerging showing that, in addition to its growth inhibitory role, p21 can function in a positive fashion toward cell proliferation (Dong et al, 2003;Dupont et al, 2003;Zhang et al, 2003), a finding, of course, with practical relevance toward cancer therapy.…”