Ptaquiloside (1) is a potent carcinogen isolated from bracken fern. Under weakly alkaline conditions, the carcinogen is converted into dienone 2 which is thought to be the ultimate agent responsible for bracken fern carcinogenicity. This study details the selective alkylation and strand scission of DNA with dienone 2. Dienone 2 forms covalent adducts through N-3 of adenine or N-7 of guanine with opening of the cyclopropyl ring. Under physiological conditions, spontaneous cleavage of the N-glycosidic linkage occurs primarily at the modified adenines to produce abasic sites. The abasic sites are so unstable that subsequent backbone breakage occurs via a beta-elimination reaction. Product analyses on sequencing gels and HPLC reveal evolution of the structures of the 5’- and 3’-termini that result from the abasic sites. In addition, the sequence selectivity for the DNA cleavage is demonstrated. The cleavage rates at the target adenine residues are affected by both 5’- and 3’-flanking nucleotides. The rank orders are 5’-AT > 5’-AG > 5’-AC > 5’-AA for 3’-flanking nucleotides and 5’AA > 5’-TA > 5’-GA > 5’-CA for 5’-flanking nucleotides (where A is a site of cleavage). The most favorable sequence is estimated to be 5’-AAAT. The present results on dienone 2 have also been compared with those of CC-1065 containing a similar reactive cyclopropyl ring.