[Oncogenic viruses and their role in tumour formation].

Oncogenic viruses trigger persistent infections, which can stimulate uncontrolled cell growth by inducing cell transformation. Different oncogenic viruses use different mechanisms for infecting cells. Most oncogenic DNA viruses integrate transforming sets of genes into the host chromosome and encode proteins that bind and inactivate cell growth regulatory proteins, such as p53 and retinoblastoma gene product. Tumourous RNA viruses use different oncogenic mechanisms. Some of them encode oncogenic proteins that are almost identical to the cellular proteins involved in the control of cellular growth. The overproduction or altered function of these oncogenic materials stimulates cell growth. These RNA viruses can cause tumours rapidly. The second group of oncoviruses integrates their promoter sequences and viral enhancers near to the cellular growth-stimulating gene, initiating the transformation of the cell. The third group of RNA tumour viruses encodes a protein tax that transactivates the expression of cellular genes. Virus-induced malignant transformation of the cell represents the first step in the complex process of oncogenesis.