Telomerase reverse transcriptase (abbreviated to TERT, or hTERT in humans) is a catalytic subunit of the enzyme telomerase, which, together with the telomerase RNA component (TERC), comprises the most important unit of the telomerase complex. Telomerases are part of a distinct subgroup of RNA-dependent polymerases. Telomerase lengthens telomeres in DNA strands, thereby allowing senescent cells that would otherwise become postmitotic and undergo apoptosis to exceed the Hayflick limit and become potentially immortal, as is often the case with cancerous cells. To be specific, TERT is responsible for catalyzing the addition of nucleotides in a TTAGGG sequence to the ends of a chromosome's telomeres. This addition of repetitive DNA sequences prevents degradation of the chromosomal ends following multiple rounds of replication. hTERT absence (usually as a result of a chromosomal mutation) is associated with the disorder Cri du chat. Function Telomerase is a ribonucleoprotein polymerase that maintains telomere ends by addition of the telomere repeat TTAGGG. The enzyme consists of a protein component with reverse transcriptase activity, encoded by this gene, and an RNA component that serves as a template for the telomere repeat. Telomerase expression plays a role in cellular senescence, as it is normally repressed in postnatal somatic cells, resulting in progressive shortening of telomeres. Studies in mice suggest that telomerase also participates in chromosomal repair, since de novo synthesis of telomere repeats may occur at double-stranded breaks. Alternatively spliced variants encoding different isoforms of telomerase reverse transcriptase have been identified; the full-length sequence of some variants has not been determined. Alternative splicing at this locus is thought to be one mechanism of regulation of telomerase activity. Regulation The hTERT gene, located on chromosome 5, consists of 16 exons and 15 introns spanning 35 kb. The core promoter of hTERT includes 330 base pairs upstream of the translation start site (AUG since it's RNA by using the words "exons" and "introns"), as well as 37 base pairs of exon 2 of the hTERT gene. The hTERT promoter is GC-rich and lacks TATA and CAAT boxes but contains many sites for several transcription factors giving indication of a high level of regulation by multiple factors in many cellular contexts. Transcription factors that can activate hTERT include many oncogenes (cancer-causing genes) such as c-Myc, Sp1, HIF-1, AP2, and many more, while many cancer suppressing genes such as p53, WT1, and Menin produce factors that suppress hTERT activity. Another form of up-regulation