Abstract Despite extensive studies on the role of tumor suppressor p53 protein and its homologues, p73 and p63, following their overexpression or cellular stress, very little is known about the regulation of the three proteins in cells during physiologic cell cycle progression. We report a role for p73 and p63 in supporting cellular proliferation through the transcriptional activation of the genes involved in G1-S and G2-M progression. We found that in MCF-7 cells, p73 and p63, but not p53, are modulated during the cell cycle with a peak in S phase, and their silencing determines a significant suppression of proliferation compared with the control. Chromatin immunoprecipitation analysis shows that in cycling cells, p73 and p63 are bound to the p53-responsive elements (RE) present in the regulatory region of cell cycle progression genes. On the contrary, when the cells are arrested in G0-G1, p73 detaches from the REs and it is replaced by p53, which represses the expression of these genes. When the cells move in S phase, p73 is recruited again and p53 is displaced or is weakly bound to the REs. These data open new possibilities for understanding the involvement of p73 and p63 in cancer. The elevated concentrations of p73 and p63 found in many cancers could cause the aberrant activation of cell growth progression genes and therefore contribute to cancer initiation or progression under certain conditions. [Cancer Res 2009;69(22):8563–71]