Oncogenes are gain-of-function mutations of normal regulatory genes or proto-oncogenes. Originally discovered in retroviruses initiating a variety of animal and avian cancers, oncogenes are believed to be important contributors to human carcinogenesis. Proto-oncogenes are altered by point mutation, amplification or rearrangement. Structural alteration of proto-oncogenes leads to a quantitative or qualitative expression change of the corresponding protein product. Oncoproteins subvert signal transduction pathways at the cell surface, in the cytosol and/or in the nucleus. Together with other oncoproteins or in the absence of tumor suppressor gene products, oncogenes contribute to human cancer formation by supporting accelerated proliferation, de-regulating cell cycle control or blocking apoptosis.