Implications for future studies in humans

While an environmental link with cancer is widely accepted, its implications are not always understood. Because of the multistage nature of carcinogenesis, it is insufficient to study initiation only at the level of the 'target cell', since events before and after initiation may prove pivotal to neoplastic development. Estimates of the relative contributions of factors causing human cancer are briefly summarized. Tumours related to patterns of life style--including diet and behaviour--and individual susceptibility are discussed. It is in this group of tumours that metabolizing enzymes are probably most significant, especially for weak initiating stimuli, whether extrinsic or intrinsic. There are few epidemiological data available at present but future research can be suggested, to improve the definition in biochemical terms of individual susceptibility and effects of life style.