Oncogenes and tumor suppressor genes as paradigms in oncogenesis.

Cancer is the result of genetic and epigenetic changes that occur mainly in stem (precursor) cells of various cell types. Two main categories of genes are involved in the process of carcinogenesis. Oncogenes are activated proto-oncogenes and tumor suppressor genes are inactivated by mutation in the global sense, that is point mutation, deletion, rearrangement, and duplication. Both types of genes are required for normal cell proliferation and differentiation and aberrant expression leads to abnormal cell proliferation. Ras and p53 genes are the paradigms for oncogenes and tumor suppressor genes, respectively, whereas the oncogenes carried by the human papillomaviruses (HPV) comprise the best example of tumor viruses involvement in human cancer.