Final answer:
The two main classes of cancer genes are proto-oncogenes, which when mutated can promote uncontrolled cell division, and tumor suppressor genes, which when dysfunctional fail to inhibit cell division. An important tumor suppressor gene affected in many cancers is p53.
Step-by-step explanation:
The two main classes of cancer genes are proto-oncogenes and tumor suppressor genes. Proto-oncogenes are normal genes that, when mutated or expressed at inappropriately high levels, become oncogenes that promote the excessive division of cells, acting similarly to an overactive accelerator in cell proliferation. Tumor suppressor genes, on the other hand, are responsible for inhibiting cell division and promoting apoptosis; when they are mutated or disabled, they fail to halt the cell cycle, comparable to a malfunctioning brake. An example of a major tumor suppressor gene is p53, which is mutated in over 50 percent of all cancers and plays a critical role in controlling cell division and apoptosis.