p53 is required for downregulation of Cdk2 and Rb phosphorylation in licensing-deficient cells. (A) Whole cell extracts from control, cdc6, p53 or Cdc6 and p53 siRNA transfected NHF1 cells were probed with antibodies to detect phospho-Rb (S807/S811), phospho-Cdk2 (T160), p53, Cdc6 and tubulin. (B) HeLa and U20S cells were transfected with control or cdc6 siRNA, and whole cell extracts were subjected to immunoblot analysis with antibodies to detect phospho-Cdk2 (T160), phospho-Rb (S807/ S811), phospho-Chk2 (T68), Cdc6 and tubulin. (C) Whole cell extracts from control, cdc6, p53 or Cdc6 and p53 siRNA transfected NHF1 cells were probed for phospho-Chk2, p53, Cdc6 and tubulin. (D) Model summarizing the results described in this study.