Telomestatin is a very potent telomerase inhibitor and can be isolated from Streptomyces anulatus 3533-SV4. It selectively facilitates the formation of intramolecular G-quadruplexes, in particular, that produced from the human telomeric sequence d[T2AG3]4. Firstly, in vitro, Telomestatin promotes or stabilizes the formation of the intramolecular G-quadruplex. When the DNA concentrations are at 0.005...