Telomeres and telomerase play essential roles in the regulation of the lifespan 
of human cells. While normal human somatic cells do not or only transiently 
express telomerase and therefore shorten their telomeres with each cell 
division, most human cancer cells typically express high levels of telomerase 
and show unlimited cell proliferation. High telomerase expression allows cells 
to proliferate and expand long-term and therefore supports tumor growth. Owing 
to the high expression and its role, telomerase has become an attractive 
diagnostic and therapeutic cancer target. Imetelstat (GRN163L) is a potent and 
specific telomerase inhibitor and so far the only drug of its class in clinical 
trials. Here, we report on the structure and the mechanism of action of 
imetelstat as well as about the preclinical and clinical data and future 
prospects using imetelstat in cancer therapy.