Not only do they selectively target the telomerasepositive Vismodegib cancer cells, but their growth inhibitory effects increase as the targeted cells perform an increasing number of cell divisions. In the present study, we have characterized the effects of a telomerase inhibitor, GRN163L, on the cellular lifespan and survival of a panel of pancreatic cancer cell lines. Telomerase is the enzyme responsible for the maintenance of telomeres, essential structures that cap and protect the ends of linear chromosomes. Human telomeres are made of tandem copies of n DNA repeats and of associated proteins, which together form a protective capping complex. This cap protects chromosomal ends from degradation, interchromosomal fusions and from being recognized as double-stranded DNA breaks, a form of DNA damage. Because of problems associated with the replication of the ends of linear DNA molecules, the so-called end-replication problems, telomeres shorten each time human somatic cells divide and this attrition limits their lifespan. Once the shortest telomere become uncapped, a DNA damage response is induced that mobilizes the p53 and p16/pRB pathways, which then act together to induce senescence, a viable state of irreversible quiescence. If the p53 and p16/pRB pathways are disabled, the cells will ignore these growth inhibitory signals and will continue to divide and shorten their telomeres. Eventually, terminal telomere shortening lead to crisis, a non-viable state associated with programmed cell death. Crisis is triggered by recurrent cycles of telomeretelomere fusions, anaphase bridges and chromosome breakage. When present, telomerase can prevent the induction of senescence and crisis and extend cellular lifespan by the synthesis and addition of new telomeric repeats to the telomeres. Telomerase is ubiquitously present in the early stages of human development. But by the time of birth, expression of the enzyme is repressed and telomerase becomes absent from most somatic tissues, including the pancreas. Cancer specimens, in stark contrast to normal tissues, are LEE011 almost always positive for telomerase activity, including pancreatic ductal adenocarcinomas. Detected in more than 85% of cancers, irrespective of the tumor type, telomerase is one of the best known markers of cancer cells. Moreover, this expression of telomerase in cancer cells is required for their unlimited proliferation or immortality, a hallmark of cancer. Accordingly, the inhibition of telomerase in cancer cells leads to telomere attrition and limits the lifespan of these cells. After sufficient telomere attrition has taken place, telomerase-inhibited cancer cells will succumb to either senescence or apoptosis, depending on the cellular system. This reliance on telomerase from their unlimited growth and the almost universal expression of telomerase in cancer cells make telomerase an attractive target for cancer therapy. A potential drawback, however, are the delays needed before the targeted cancer cells have lost sufficient telomeres for senescence or crisis to be induced. This delayed action might preclude their use as a first line of treatment for cancer, but to block the regrowth of residual disease after conventional therapy, telomerase inhibitors have been expected to have good therapeutic potential. Telomere shortening is the earliest and most common genetic alteration acquired during pancreatic cancer development. This alteration, detected in 96% of PanIN precursor lesions, is accompanied by evidence of a DNA damage response consistent with telomere uncapping and dysfunction. Not surprisingly, more than 90% of these tumors eventually re-activate telomerase, which otherwise remains undetectable in normal pancreatic tissues.