How chemicals may induce cancer.

Recent studies which lead to a better understanding of chemical carcinogenesis are reviwed. Almost all carcinogens require an activation by metabolic processing in order to operate "in vivo" or "in vitro". Some chemicals and drugs induce the enzymes which metabolize carcinogens. This induction seems to be of fundamental importance to the development of human cancers resulting from environmental pollution. Nearly all the lesions in cellular DNA caused by chemical carcinogens can be repaired by a DNA repair system. Failure to repair some lesions or the action of "error prone" repair can lead to mutations. The mutagenic effect of some chemicals provides a rapid screening test for the detection of their carcinogenic activity. Such tests are described and analyzed in detail. Finally, a general and overall scheme for possible mechanisms of action of chemical carcinogens is proposed.