Human telomerase is an RNA-dependent DNA polymerase that synthesizes telomeric DNA sequences and is essential for cellular immortality. It consists of two essential components: the RNA component (hTR or hTERC) that serves as a template for telomeric DNA synthesis, and the catalytic protein (hTERT) with reverse transcriptase activity. hTR is highly expressed in all tissues, while hTERT is closely associated with telomerase activity and is generally repressed in normal cells and upregulated in immortal cells. Telomerase activity is absent in most normal human somatic cells but present in over 90% of cancerous cells and in vitro-immortalized cells. Telomerase plays a crucial role in cellular aging and tumorigenesis, as its regulation is important for developing diagnostic and therapeutic strategies. Telomerase activity can be reconstituted in vitro using hTR and hTERT, indicating that these components are essential for telomerase function. However, telomerase activity does not always imply telomere maintenance in vivo. The functional assembly of telomerase involves other components and associated proteins to form a functional, multisubunit telomerase holoenzyme. The RNA component (hTR) provides the template for telomeric repeat synthesis and has a conserved secondary structure with telomerase RNAs from various vertebrate species. The catalytic subunit (hTERT) is essential for telomerase activity and is involved in telomere elongation. hTERT is regulated by various factors, including transcriptional regulation, posttranslational modifications, and interactions with other proteins. Telomerase activity is regulated at multiple levels, including transcription, mRNA splicing, maturation and modifications of hTR and hTERT, transport and subcellular localization of each component, assembly of active telomerase ribonucleoprotein, and accessibility and function of the telomerase ribonucleoprotein on telomeres. Transcriptional regulation of hTERT is a primary and rate-limiting step in the activation of telomerase activity in most cells. The hTERT promoter has been characterized and is involved in the regulation of hTERT expression. Several transcription factors, including c-Myc, Sp1, and human papillomavirus 16 E6, are involved in the activation of hTERT transcription. Steroid hormones, such as estrogen and progesterone, also regulate hTERT transcription and telomerase activity. Negative regulators of hTERT transcription include Mad1, p53, pRB and E2F, Wilms' tumor 1 tumor suppressor, and Myeloid cell-specific zinc finger protein 2. These regulators play a role in the repression of hTERT transcription and telomerase activity in normal cells. The regulation of telomerase activity is essential for understanding the mechanisms of cellular aging and tumHuman telomerase is an RNA-dependent DNA polymerase that synthesizes telomeric DNA sequences and is essential for cellular immortality. It consists of two essential components: the RNA component (hTR or hTERC) that serves as a template for telomeric DNA synthesis, and the catalytic protein (hTERT) with reverse transcriptase activity. hTR is highly expressed in all tissues, while hTERT is closely associated with telomerase activity and is generally repressed in normal cells and upregulated in immortal cells. Telomerase activity is absent in most normal human somatic cells but present in over 90% of cancerous cells and in vitro-immortalized cells. Telomerase plays a crucial role in cellular aging and tumorigenesis, as its regulation is important for developing diagnostic and therapeutic strategies. Telomerase activity can be reconstituted in vitro using hTR and hTERT, indicating that these components are essential for telomerase function. However, telomerase activity does not always imply telomere maintenance in vivo. The functional assembly of telomerase involves other components and associated proteins to form a functional, multisubunit telomerase holoenzyme. The RNA component (hTR) provides the template for telomeric repeat synthesis and has a conserved secondary structure with telomerase RNAs from various vertebrate species. The catalytic subunit (hTERT) is essential for telomerase activity and is involved in telomere elongation. hTERT is regulated by various factors, including transcriptional regulation, posttranslational modifications, and interactions with other proteins. Telomerase activity is regulated at multiple levels, including transcription, mRNA splicing, maturation and modifications of hTR and hTERT, transport and subcellular localization of each component, assembly of active telomerase ribonucleoprotein, and accessibility and function of the telomerase ribonucleoprotein on telomeres. Transcriptional regulation of hTERT is a primary and rate-limiting step in the activation of telomerase activity in most cells. The hTERT promoter has been characterized and is involved in the regulation of hTERT expression. Several transcription factors, including c-Myc, Sp1, and human papillomavirus 16 E6, are involved in the activation of hTERT transcription. Steroid hormones, such as estrogen and progesterone, also regulate hTERT transcription and telomerase activity. Negative regulators of hTERT transcription include Mad1, p53, pRB and E2F, Wilms' tumor 1 tumor suppressor, and Myeloid cell-specific zinc finger protein 2. These regulators play a role in the repression of hTERT transcription and telomerase activity in normal cells. The regulation of telomerase activity is essential for understanding the mechanisms of cellular aging and tum