Initiators, promoters and the uncertainty principle.

The development of cancer in humans and in animals including experimental animals is almost always a multistep process. With carcinogenic chemicals and probably with some other etiological agents as well (e.g. radiations, DNA viruses), the first few steps require exogenous stimuli to induce the requisite tissue changes. However, the known agents are now being shown to have different and even antagonistic or contradictory effects depending upon when and how they are used. This uncertainty principle in regard to agents in cancer development, although complicating the analysis, may facilitate a more rational scientific analysis of the fundamental nature of the different steps as cancer develops and could lead to new approaches to cancer prevention and/or therapy.