Sir Francis Crick said: “DNA is, in fact, so precious and so fragile that we now know that the cell has evolved a whole variety of repair mechanisms to protect its DNA from assaults by radiation, chemicals and other hazards. This is exactly the sort of thing that the process of evolution by natural selection would lead us to expect” (1988, What Mad Pursuit. Basic Books: New York). As Crick pointed out, damage to DNA sparks a highly regulated and strictly controlled process in a normal cell. If some of these processes are interrupted, it may result in neoplasm and tumour growth. The main objective of this work has been to investigate genetic alterations in two genes encoding proteins involved in cell cycle control and the DNA damage response machinery. The ATMa (Ataxia Telangiectasia Mutated) gene is analyzed for mutations in AT (Ataxia Telangiectasia) patients and their relatives to estimate the cancer risk for ATM heterozygous individuals. These data are also included in a large Nordic epidemiologic study. The TP53 gene is analyzed for a polymorphism in patients with colon and breast cancer to assess whether this polymorphism is associated with mutated TP53 in the cancers.