Abstract Environmental agents (chemicals, radiation and viruses) can be carcinogens (cancer‐causing substances) and mutagens (substances that alter DNA). The steps from mutagen/carcinogen to tumour involve mutagen/carcinogen activation by metabolism, reaction with DNA to give a DNA adduct and DNA replication of the adduct, resulting in a mutation; a mutation in a cancer gene may result in enhanced growth potential, and ultimately in tumour growth.