Mammalian intracellular ribonuclease L (RNase L) is a latent endoribo-nuclease that functions against viral infections as an apoptosis-inducing protein, and its activity requires intracellular 5¢-end-triphosphorylated-2¢,5¢ oligoadenylates (2-5A) as an activator. Previously, we showed that RNase L can be activated in human cancer cell line HT1080 by an RNA polymerase I inhibitor, 1-(3-C-ethynyl-b-d-ribo-pentofuranosyl)cytosine (3¢-ethynylcytidine; ECyd).