The pyrrolobenzodiazepines (PBDs) are a class of antitumor antibiotics. PBDs are sequence-selective DNA minor-groove binding crosslinking agents originally discovered in Streptomyces species and are significantly more potent than systemic chemotherapeutic drugs. Pyrrolobenzodiazepines are potent inhibitors of nucleic acid synthesis because of their ability to recognize and bind to specific sequences of DNA and form a labile covalent adduct.