CDKN1A and neoplasm: Substantial reduction of tumour growth was observed in p16/p21-DKO mice but not in mice lacking either p16Ink4a or p21Cip1/Waf1 (single KO mice) (Fig. 2a, b; Supplementary Fig. 2), suggesting that both CDK2 and CDK4 associate with this transcription factor in a complementary fashion.