A telomere is a region of repetitive DNA at the end of a chromosome (TAAGGG), which protects the end of the chromosome from deterioration. Its name is derived from the Greek nouns telos (τέλος) "end" and merοs (μέρος, root: μερ-) "part". The telomere regions deter the degradation of genes near the ends of chromosomes by allowing for the shortening of chromosome ends, which necessarily occurs during chromosome replication.

It has been suggested that DNA sequences would be lost in every replicative phase until they reached a critical level, at which point cell division would stop. It is assumed that at birth telomeres are consisted of 10.000 base-pares, whereas in elderly people they are shortened up to 500 base-pairs.

Telomeres protect a cell's chromosomes from fusing with each other or rearranging — abnormality that can lead to cancer. Characteristic of cardiovascular diseases and cancer is telomere instability, which can be analyzed and estimated from blood leukocytes.