THE success of cisplatin in cancer chemotherapy derives from its ability to crosslink DNA and alter the structure, Most cisplatin-DNA adducts are intrastrand d(GpG) and d(ApG) crosslinks(1), which unwind and bend the duplex to facilitate the binding of proteins that contain one or more high-mobility-group (HMG) domains(2). When HMG-domain proteins such as HMG1, IXR (intrastrand-crosslink recognition) protein from yeast, or human upstream-binding factor (hUBF) bind cisplatin intrastrand crosslinks, they can be diverted from their natural binding sites on the genome and shield the adducts from excision repair(3-5). These activities sensitize cells to cisplatin and contribute to its cytotoxic properties, Crystallographic information about the structure of cisplatin-DNA adducts has been limited to short single-stranded deoxyoligonucleotides such as Cis-[Pt(NH3)(2){d(pGpG)}](6-8). Here we describe the X-ray structure at 2.6 Angstrom resolution of a double-stranded DNA dodecamer containing this adduct. Our information provides, to our knowledge, the first crystallographic look at a platinated DNA duplex and should help the design of new platinum and other metal crosslinking antitumour drug candidates. Moreover, the structure reveals a unique fusion of A- and B-type DNA segments that could be of more general importance.