Human somatic cells gradually lose their telomeric repeats each cell division, and when they become critically shortened, stop dividing. On the other hand, in immortal cancer cells and germline cells, telomerase, a ribonucleoprotein which can compensate for the loss of telomeric repeats synthesizing telomeric DNA onto chromosomal ends, is activated and the telomere lengths are stabilized. Thus, telomere length and telomerase activity are believed to be characteristic indicators of cell proliferation and cell immortality, and inhibition of telomerase activity is expected to be a new strategy of anti-cancer therapy.