It has been estimated that exposure to environmental chemical carcinogens may contribute significantly to the causation of a sizable fraction, perhaps a majority, of human cancers. Human carcinogens act through a variety of genotoxic and non-genotoxic mechanisms. Genotoxic carcinogens can attack biological macromolecules such as DNA and RNA either directly or indirectly through metabolism, resulting in the formation of adducts with these macromolecules. If DNA adducts escape cellular repair mechanisms and persist, they may lead to miscoding, resulting in permanent mutations. Mutations cause an undefined number of cell changes, translated into aberrant protein expression and in changes in cell cycle control.