While the present and previous studies demonstrate that in several frequently-studied p53-proficient cancer cell lines (HCT-116, U2OS, MCF7) CycB1-Cdk1 complexes are minor p21 targets16,21,23, we cannot exclude that p21 contributes to G2 arrest by inhibiting Cdk1 associated with CycA. Here, TP53 is linked to cancer.