Telomere is the structure which is located on both ends of individual chromosomes in eukaryotes. The DNA sequence of the telomere consists of Guanine-rich tandem repeat, i.e., (TTAGGG)n in man. Telomere protects the end of the chromosome from fusion or deletion and maintains the stability of the chromosome and is synthesized by telomerase, a ribonucleoprotein. Telomere reduction is observed with cell senescence and immortalization, both in vivo and in vitro. Thus, telomere is considered to be a "clock" which measures the life span of cells, and its length is altered by cellular senescence and immortalization.