The end of chromosome in higher eukaryote are termed telomere. Telomeric DNA consists of tandemly repeated DNA sequences which are gradually lost with age and with replication in vitro .Telomerase is a specialized reverse transcriptase that catalyzes elongation of the telomeric tandem repeat, TTAGGG, by addition of this sequence to the ends of existing telomeres.Telomerase is expressed in embryonic cells and in adult germline cells, but it is undetectable in normal somatic cells except for proliferative cells of renewable tissues.It has been discovered that there is an association between telomerase activity and cell proliferation because terminal differentiation is followed by down regulation of telomerase activity.Also, there are recent observations that cellular life-span can be extended by forced expression of telomerase, this might provide a route to delay some of the aged related changes ascribable to replicative senescence.The normal hemopoietic system harbors telomerase competent cells with the capacity to upregulate telomerase activity to a notable levels. High telomerase activity is found in progenitor stem cells and activated lymphocytes in vitro as well as in vivo, indicating that cells with high growth requirement can upregulate telomerase activity.Also, telomerase activity can be detected in most primary human specimens and tumor derived cell lines. The activation of telomerase in malignant cancers seems to be an important step in tumorigenesis, whereby the cell gain the ability of indefinite proliferation and become immortal.In malignant hematopoietic disorders telomerase activity is a general finding but with a different levels. The highest telomerase expression has been shown in acute leukemias and non-Hodgkin's lymphomas also the level of activity is parallel to tumor progression, also the mean TRF length may disclose how far the disease has progressed and may be a marker for disease severity.Thus, it is becoming generally accepted that telomerase could be a reliable new marker for detection of cancers. Moreover, a novel therapy for tumor expression has been developed using telomerase inhibitors, with the hope that the lack of telomerase expression in normal somatic cells would results in highly specific treatment with fewer side effects than conventional chemotherapy