Telomeres are specialized nucleoprotein structures that cap linear eukaryotic chromosomes and function to prevent the chromosomes from recombining or unraveling (1 ). Telomerase is a ribonucleoprotein (RNP) polymerase that synthesizes telomeric sequencede novoonto the 3′ end of chromosomes using a portion of its internal RNA component as a template (2 ,3 ). The regulation of telomerase activity is complex. While most differentiated somatic cells lack telomerase activity, most cancer cells, germline cells, and stem cells express detectable levels of telomerase activity (4 ). In those cells that lack telomerase activity, telomeric sequences are incompletely replicated during cell division due to the end replication problem of DNA polymerases (5 ,6 ) and thus the net length of telomeres in the daughter cells is reduced (7 ). The progressive loss of telomeric DNA triggers a growth-arrest mechanism in normal somatic cells (8 ). However, in cells that maintain telomerase activity, telomeres can be completely replicated with no loss of sequence information during cell division, and proliferation can continue without activating this check point (9 ). Therefore, by maintaining the integrity of telomeres, telomerase plays an important role in unlimited cellular proliferation.
