The E2F family of transcription factors is believed to have an essential role in the control of cellular proliferation by regulating the transcription of genes involved in cell cycle progression. Previous work has demonstrated that the targeted inactivation of E2f1, E2f2, and E2f3 results in elevated p21(CIP1) protein levels, loss of E2F target gene expression, and cell cycle arrest at G1/S and G2/M, suggesting a strict requirement for these E2Fs in the control of normal cellular proliferation. We now demonstrate that E2f1, E2f2, and E2f3 are also required for oncogene-mediated transformation of mouse embryonic fibroblasts. Analysis of synchronized populations of mouse embryonic fibroblasts revealed that the inactivation of p21(CIP1) restores the ability of E2f1-3-deficient cells to enter and transit through G1/S (but not G2/M). In contrast, loss of p53 restored the ability of these cells to progress through both G1/S and mitosis, leading to their continued proliferation. The inactivation of p53 (but not p21(CIP1)) rendered E2f1-3-deficient cells sensitive to transformation and tumorigenesis. These results suggest that the negative regulation of the p53-p21(CIP1) axis by the E2F1-3 factors is critical for cell cycle progression and cellular transformation.