Dactinomycin
Dactinomycin is a chromopeptide antineoplastic antibiotic isolated from the bacterium Streptomyces parvulus.  Dactinomycin intercalates between adjacent guanine-cytosine base pairs, blocking the transcription of DNA by RNA polymerase; it also causes single-strand DNA breaks, possibly via a free-radical intermediate or an interaction with topoisomerase II. (NCI04)