A novel telomerase inhibitor, telomestatin, isolated from Streptomyces anulatus 
is the most potent telomerase inhibitor so far. Telomestatin specifically 
inhibited telomerase without affecting reverse transcriptases and polymerases. 
In addition, telomestatin induced telomere shortening, but its ratio was 
extremely faster than that observed in physiological telomere shortening. These 
results suggested the existence of other mechanisms to inhibit telomerase. 
Telomeres consist of guanine rich sequences which compose a characteristic 
three-dimensional structure designated as G-quadruplex. Stabilization of 
G-quadruplex structure inhibited the catalysis of not only telomerase but also 
other DNA interacting molecules. Telomestatin potently stabilized G-quadruplex 
structure in a specific manner. G-quadruplex structure is also involved in a lot 
of oncogene promoters. Thus, telomestatin provide the novel therapeutic 
molecular target for cancer chemotherapy.