The subtleties of the structure and function of bleomycin A2 (1), a clinically employed antitumor agent that derives its biological properties through the sequence‐selective cleavage of DNA in a process that is dependent on the metal ion and O2, have been unraveled in a stepwise manner. Systematic modifications in the structure of 1 enabled many of the subtle functional roles of the individual subunits and their substituents in the efficiency, selectivity, and preference (double strand versus single strand) of DNA cleavage to be elucidated.