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A definition of the term "telomerase" is presented. It refers to a ribonucleoprotein enzyme complex that maintains chromosome ends and has been referred to as a cellular immortalizing enzyme. It is composed of both RNA and proteins. It can be seen in most fetal tissues, normal adult male germ cells, inflammatory cells, and in most tumor cells.

A definition of the term "telomerase" is presented. It refers to the enzyme which maintains and stabilizes the telomeres in growing tissue to prevent their shortening and disappearance that could lead to the cessation of cell division in cells. Telomerase are expressed most in cancer cells rather than differentiated cells.

A definition of the term "telomeric repeat amplification protocol" is presented. It refers to a highly sensitive polymerase chain reaction-based assay for measuring telomerase activity.

A definition of the term "TERT," the acronym for telomerase reverse transcriptase, is presented. It refers to the telomerase catalytic subunit of 1132 amino acids and 126 kD. The human TERT, also known as TRT, TP2, TCS1 or hEST2, gene locus maps at 5p15.33 and the mouse tert gene locus at chromosome 13. TERT is a reverse transcriptase that adds simple sequence repeats to chromosome ends by copying a template sequence within the RNA component of the enzyme.

A definition of the term "hTERT," which is the acronym for human telomerase reverse transcriptase that is a catalytic subunit of telomerase, is presented.

A definition of the term "alternative lengthening of telomeres" (ALT) is presented. It refers to a telomere maintenance mechanism that does not involve telomerase. It is found in a minority of cancers and immortalized cell lines. Cells that maintain their telomeres by ALT have very heterogenous telomere lengths, ranging from undetectable to extremely long.