ABSTRACT
The discovery of naturally existing factors inhibiting proliferation of cells creates new bases for understanding the neoplastic processes that carcinogenesis may generate not only by the action of various carcinogens but also because of the loss of the inhibitory factors. The certain viruses the case the sole infection that damages the host's deoxyribonucleic acid (DNA) is insufficient to generate cancers; tumor promoters are also needed. The viruses include Retroviruses, Adenoviruses, Papovaviruses, and Herpesviruses. Recent investigations on transcription control by viral oncogenes shifted from the analysis of the sequences necessary for promoter function to proteins that stimulate transcription from a promoter in trans, named trans-activation. In higher organisms the regulatory mechanisms involve extracellular messenger molecules, including hormones and peptide growth factors together with specific cellular receptors that bind these molecules. The autonomous nature of malignant cell growth has been known for many years. Tumor necrosis factor is also a potent stimulatory factor for proliferation of certain types of normal cells, mainly fibroblasts.
