A reverse transcriptase that lengthens a structure called the telomere. It consists of a protein component and an RNA component. The RNA component has been patented by Geron Corporation, U.S. Patent No. 5,583,016. To the extent that shortening of the telomere is responsible for programmed cell death, telomerase can be said to convey cellular immortality.

Note that mice, which are very short-lived and cancer prone, express telomerase in all the cells of their body. In humans, telomerase is only expressed in 2 places, the gonads and in cancer cells. This enzyme, therefore, is a promising target for a cure for cancer.

Inhibition of telomerase has been attempted as an anti-cancer therapy by insertion of antisense rna into cancer cells. In the early stages of cancer, this proves effective - the telomeres shorten sufficiently that p53 is activated and apoptosis is initiated. However, cancer cells generally lose p53 functionality as they become more malignant. Once p53 activity has been lost, inhibiting telomerase doesn't induce apoptosis. Instead, you start losing the ends of your chromosomes. The chromosomal instability that ensues actually makes matters worse, and the tumour develops more quickly and becomes significantly more nasty. As a cure for cancer, telomerase inhibition leaves something to be desired but could be useful if the cancer is noticed soon enough.