The equine estrogens, equilin and equilenin, are major components of the drug Premarin, the most widely used formula for hormone replacement therapy. The derivative 4-hydroxyequilenin(4-OHEN), a major phase I metabolite of equilin and equilenin, autoxidizes to potent cytotoxic quinoids that can react in vitro and in vivo with cytosine and adenine in DNA. Unique cyclic adducts containing the same bicyclo[3,3,1]nonane type connection ring are produced. Each base adduct has four stereoisomers. In order to elucidate the structural effects of A versus C modification, we have carried out molecular dynamics simulations of the stereoisomeric 4-OHEN-A adducts in DNA 11-mer duplexes and compared results with an earlier study of the C adducts (Ding, S