The chromosome ends are specialized nucleoprotein structures called telomeres, which are essential for stable chromosome maintenance. In tumor-derived cell lines telomeres are maintained by the ribonucleoprotein enzyme telomerase. Telomerase activity is repressed in almost all normal human somatic cells. Due to the end replication problem, progressive telomere shortening occurs in normal somatic cells, leading to a limited replicative capacity and eventually resulting in cellular senescence. In the presence of viral oncogenes or some somatic mutations that block cellular senescence, cells continue to divide and telomere erosion continues. This continuing telomere erosion ultimately leads to the activation of telomerase, a necessary event for the sustained growth of most human tumors.