Proto-oncogenes normally promote cell division, and mutations in them which lead to cancer turn their expression up or make them more active. In contrast, tumor-suppressor genes normally work as brakes on cell division, and cancer-promoting mutations in these genes turn them off or down, allowing excess cell division.
Work Step by Step
Both of these kinds of genes, when mutated, lead to excess cell division and cancer. However, their activity is changed in opposite directions to get the same, or at least a similar, result.