Bleomycin-mediated DNA cross-links are dependent on closed-circular molecules with superhelical turns.

Non-covalent intermolecular DNA cross-links are created by reaction with the antitumor antibiotic, bleomycin. The cross-links are observed only when the reactant covalently closed circular duplex DNA contains either positive or negative superhelical turns. Multiple sites of cross-linking often extend over 500 base-pairs or more in length.