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for this reason, they can be uncovered by treatment with a tumor promoter even after a long delay.It is still not certain how tumor promoters work, and different promoters arelikely to work in different ways.

One possibility is that they simply provoke theexpression of proliferation-inducing genes that had been mutated before thepromoter was applied but were not expressed:a mutation that makes a geneproduct hyperactive will not show its effects until the gene is expressed.Alternatively, the tumor promoter may temporarily alter the way the cell reactsto theproduct of the mutated gene, either by releasing the cell from a counteractinginhibitory influence or by triggering production of a co-factor necessaryfor proliferation of the mutated gene product.

lVhichever the mechanism, the result isthat the mutant cell is enabled to grow and divide and produce a large cluster ofcells (Figure 20-24).A tlpical papilloma might contain about 105cells. If exposure to the tumorpromoter is stopped, almost all the papillomas regress,and the skin regains alargely normal appearance.In a few of the papillomas, however, further changesoccur that enable cell growth and division to continue in an uncontrolled way,even after the promoter has been withdrawn. These changes seem to originatein an occasional single papilloma cell, at about the frequency expectedfor spontaneous mutations. In this way, a small proportion of the papillomas progresstobecome cancers.Thus, the tumor promoter apparently favors the developmentof cancer by expanding the population of cells that carry an initial mutation: themore such cells there are and the more times they divide, the greater is thechance that at least one of them will sustain another mutation, or an epigeneticchange, that carries it one step further toward malignancy.Although naturally occurring cancers do not necessarily arise through thespecific sequenceof distinct initiation and promotion stepsjust described, theirevolution must be governed by similar principles.

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