Telomeres are specialized nucleoprotein complexes that protect chromosome ends from degradation and ligation,essential for both chromosomal stability and long-term cellular proliferation.Extensive studies indicate that telomerase is not active in most somatic tissues, but is expressed in approximately 90% of human cancers.The beginning of 21 st century, the relationship between telomeres and telomerase expression in cancer was not very clear.We have measured them at the same time in human gynecological cancer tissue.As a result, we did not find any relationship between telomere length and telomerase activity.Essential change is dependent on the expression of telomerase.Over the past decade, the structure and function of telomerase has been studied in more detail.Telomerase is primarily composed of a catalytic subunit (hTERT) and a RNA template (hTR or hTERC).The regulation of human telomerase activity likely involves multiple mechanisms, with a major regulatory step at hTERT transcription.In cancer cells and immortalized cell lines, the level of hTERT expression has been shown to correlate with the presence of telomerase activity.Several transcription factors such as c-Myc and Spl regulating the hTERT transcription have been identified.Histone deacetylase (HDAC) inhibitors can inhibit the proliferation and induce cell death in a wide variety of transformed cells.In recent studies, the HDAC inhibitors induced hTERT transcription and significantly activated the expression of hTERT mRNA and telomerase activity in normal cells.However, in cancer cells, the HDAC inhibitors exhibited different effects on the hTERT mRNA expression and telomerase activity according to the type of cancer cells.We examined the effects of HDAC inhibitors in ovarian cancer cell line.As a result, HDAC inhibitors inhibited cell proliferation and induced apoptosis, but had no effect on the expression of hTERC and hTERT mRNA and on telomerase activity.Currently, several clinical trials using telomerase inhibitors and telomerase therapeutic vaccines have been completed or in progress.From these results, new findings on the expression of human telomerase would be obtained.Cancer treatment by control of telomerase has just begun.