Members of the Fanconi anemia (FA) protein family are involved in repair of genetic damage caused by DNA cross-linkers. It is not clear whether the FA proteins function in oxidative DNA damage and oncogenic stress response. Here, we report that deficiency in the Fanca gene in mice elicits a p53-dependent growth arrest and DNA damage response to oxidative DNA damage and oncogenic stress. Using a Fanca^−/−Trp53^−/− double knockout model and a functionally switchable p53 retrovirus, we define the kinetics, dependence, and persistence of p53-mediated response to oxidative and oncogenic stresses in Fanca^−/− cells. Notably, oxidative stress induces persistent p53 response in Fanca^−/− cells, likely due to accumulation of unrepaired DNA damage. On the other hand, whereas wild-type cells exhibit prolonged response to oncogene activation, the p53-activating signals induced by oncogenic ras are short-lived in Fanca^−/− cells, suggesting that Fanca may be required for the cell to engage p53 during constitutive ras activation. We propose that the FA proteins protect cells from stress-induced proliferative arrest and tumor evolution by acting as a modulator of the signaling pathways that link FA to p53. [Cancer Res 2008;68(23):9693–702]