DNA damage is the source of pro-cancerous mutations. In addition, recent evidence has suggested that the reverse connection might also exist ; namely that oncogenes can promote the generation of DNA damage. However, the nature of the damage that is caused by oncogenes is still poorly understood. Our laboratory has centred its research on trying to understand how cells respond to ‘replicative stress’ (RS), a type of DNA damage that unavoidably occurs every time that a cell replicates its DNA, and which is mainly prevented by ATR and Chk1 kinases. Unfortunately, the essential nature of these kinases poses important limitations on their study, particularly at the organism level. In order to overcome these limitations, a significant part of our work over the last few years has been focused on the development of cellular and animal tools for the study of ATR and Chk1. These tools include mice with enhanced or limited ATR-Chk1 function, cell systems in which the pathway can be activated at will, and chemical inhibitors of the ATR kinase. Our studies have revealed the impact of replication stress on cancer and ageing, and have provided drugs that can be used to test our ideas on how to approach cancer therapy. Altogether, our main aim is to understand how genome maintenance is safeguarded – particularly during replication – and to exploit this knowledge as a way to fight against cancer.