Telomerase is an enzyme which is able to elongate telomeres and repair short telomeres by re-elongating them. To this end, telomerase adds telomeric repeats to the chromosome ends. In non-pathological conditions telomerase is expressed in early stages of embryo development as well as in certain adult stem cell compartments. Telomerase is also highly expressed in pathological conditions, such as cancer, where it sustains the immortal growth of cancer cells. Healthy cells usually produce little or no telomerase and, as a consequence of this, they progressively shorten
their telomeres associated with successive cycles of cell division, until they reach a critically short length which triggers cell death or an irreversible cell arrest known as replicative senescence (also known as the Hayflick limit).