Cells have evolved sophisticated mechanisms to maintain genomic stability after cellular stress. Activation of DNA damage response pathways, and most importantly p53, leads to adaptive responses that can be influenced by different extracellular factors. The aim of this project was to study how extracellular factors modulate p53 cell-fate decisions after DNA damage, with particular interest in oxygen tensions and vitamin A metabolites. First, we focused on how physiological oxygen tensions (5% 02) may influence cellular responses to genotoxic stress. We showed that normal and cancer cells cultured at 5% 02 had a reduction in p53-mediated apoptosis after exposure to different genotoxic stresses. This was not due to a decrease p53 protein levels or its transactivation activity, and the oxidative damage caused by DNA damaging agents was not affected by oxygen tensions. We also found a p53-independent activation of MAPK at 5% 02, which when inhibited restored levels of p53-induced apoptosis. HIF-1α, a transcription factor induced at lower O2 concentrations, was present and active at 5% 02. However, this did not affect MAPK activation and HIF-1α was not involved in the resistance to apoptosis under these conditions, although MAPK was necessary for HIF-1α expression and activation. We next explored the effect of the vitamin A (retinol) pathway on the cellular responses to DNA damage. We showed that Stra6, a retinol-inducible gene, is upregulated by p53 after DNA damage. While overexpression of Stra6 sensitized cells to p53-induced apoptosis independently of the retinoic acid signalling, its inhibition resulted in decreased apoptosis after DNA damage and less induction of oxidative stress. This shows that both oxygen tensions and vitamin A metabolites, through Stra6, are potent modulators of the p53 responses to DNA damage.