A carcinogen is any substance, radionuclide or radiation, that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes.

Source: Wikipedia

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Chemicals are defined as carcinogenic if they induce tumours, increase tumour incidence and/or malignancy or shorten the time to tumour occurrence. Traditionally, carcinogens have been identified from epidemiological studies or from animal experiments. Carcinogenic chemicals have conventionally been divided into two broad categories based of the presumed mode of action: genotoxic or non-genotoxic. Genotoxic carcinogens cause damage by interacting directly with DNA – many known mutagens are in this category. In contrast, non-genotoxic carcinogens cause “epigenetic” changes, i.e. effects that do not involve alterations in DNA but that may influence the carcinogenic process. The mechanistic understanding of the carcinogenic process differs considerably between the two modes of action. The distinction is not absolute – chemicals can be carcinogenic by both models of action.

Source: JRC Technical Report Review of QSAR Models and Software Tools for Predicting Genotoxicity and Carcinogenicity EUR 24427 EN - 2010.