Different types of carcinogens and their possible modes of action: a review.

Of the many aspects of cancer research, three which are perhaps of particular interest to chem ists are the problems of cell metabolism, of chemo therapy, and of carcinogenesis. All these aspects are closely related, and at the present stage of de velopment of the subject chemotherapy and car cinogenesis are intimately connected, as most of the physical and chemical agents used in therapy of cancer are themselves carcinogenic. The correla tion between these two properties has been so close in the past few years that, when a new agent is in troduced for therapy, one is immediately sus picious that it may be carcinogenic. From the human point of view, the two aspects of the cancer problem which are most important are prevention and cure. Experiments on chemo therapy are likely to lead to control of the disease and a study of carcinogenesis to prevention or re duction of the incidence. The investigation of car cinogenesis might reduce the incidence of the dis ease in at least two ways. One of these is the identi fication of carcinogens which might be acting as extrinsic carcinogenic agents in the external en vironment. It is obvious that any known car cinogens should be avoided, but they may be dif ficult to recognize. One approach to carcinogenesis is through the examination of the incidence of can cer in sections of the population. In the case of chimney-sweeps and the early operators of x-ray