Oncogenes are altered (mutated) cellular genes associated with cellular transformation and cancer (Table 23.7). The unmutated cellular form of an oncogene is called a proto-oncogene. Proto-oncogenes can act as oncogenes too if a mutation controlling the expression of that particular gene affects either the amount or timing of the gene's expression. Oncogenes transferred to a cell by a virus are called viral oncogenes. Viral oncogenes are usually mutated forms of normal cellular genes that probably arise from rare genome excision events (Figure 23.19).

Oncogenes can be placed into several categories, as follows:

1. Protein kinases;

2. G proteins;

3. Nuclear receptors;

4. Growth factors;

5. Growth factor receptors; and

6. Tumor Suppressors.