The anticancer activity of cis-diamminedichloroplatinum(II) (cisplatin) arises from its ability to damage DNA, with the major adducts formed being intrastrand d(GpG) and d(ApG) crosslinks(1). These-crosslinks bend and unwind the duplex, and the altered structure attracts high-mobility-group domain (HMG) and other proteins(2). This binding of HMG-domain proteins to cisplatin-modified DNA-has been postulated to mediate the antitumour properties of the drug(3,4). Many HMG-domain proteins recognize altered DNA structures such as four-way junctions and cisplatin-modified DNA(5), but until now:the molecular basis for this recognition was unknown. Here we describe mutagenesis, hydroxyl-radical footprinting and X-ray studies that elucidate the structure of a 1:1. cisplatin-modified DNA/HMG-domain complex Domain A of the structure-specific HMG-domain protein HMG1 binds to the widened minor groove of a 16-base-pair DNA duplex containing a site-specific cis-[Pt(NH3)(2){d(GpG)-N7(1),-N7(2)}] adduct, The DNA is strongly kinked at a hydrophobic notch created at the platinum-DNA crosslink and protein binding extends exclusively to the 3＇ side of the platinated strand. A phenylalanine residue at position 37 intercalates into a hydrophobic notch created at the platinum crosslinked d(GpG) site and binding of the domain is dramatically reduced in a mutant in which alanine is substituted for phenylalanine at this position.