SV40-Mediated Immortalization

Normal cells in culture divide a defined number of times before reaching replicative senescence (1 ). The number of divisions that cells can undergo varies with the species of animal and tissue from which the cells are derived. Cells that have exceeded their typical number of divisions are said to have an extended life-span. Cells that have permanently bypassed the natural senescence point are said to have been immortalized. Immortalization can be brought about by the accumulation of mutations in cellular genes (spontaneous immortalization) or by the introduction of specific types of oncogenes, including T antigen (T-ag).