Carcinogenesis: molecular defences against carcinogens.

It has long been recognized that exposure to specific chemical and physical agents can result in the appearance of tumours in both man and animals. There is substantial evidence that DNA is the ultimate cellular target of carcinogens and that DNA repair is an important cellular defence against the effects of carcinogen exposure. Some insight into the molecular processes involved in cellular responses to carcinogens is provided by the occurrence of rare human disorders that display complex abnormalities in processing DNA damage or maintaining genomic integrity. These disorders include xeroderma pigmentosum, Cockayne syndrome, ataxia telangiectasia, dysplastic nevus syndrome and Bloom syndrome. Such disorders have provided clues to some of the basic mechanisms involved in carcinogenesis.