[Mechanisms of activation and function of oncogenes]

Transforming genes (oncogenes) were first identified and characterized as genetic elements of tumorviruses. It has been demonstrated that these genes are derived from cellular genes by transduction. The corresponding cellular genes are ubiquitous in nature and have been well conserved during evolution. It is therefore very likely that they are important elements in normal regulation of cell growth, e.g. during development. Several mechanisms of oncogene activation have been observed. Qualitative as well as quantitative changes in oncogene expression lead to cell transformation. According to their mechanism of action, at least 4 functional groups of oncogenes can be distinguished. It has recently been shown that the transformation of a normal cell into a tumor cell proceeds in several steps involving different oncogenes.