The clinical activity of the antitumor drug cis-diamminedichloroplatinum(II) (cis-DDP) is thought to be related to its ability to form bifunctional lesions in DNA. trans-Diamminedichloroplatinum(II) (trans-DDP), the stereoisomer of cis-DDP, is clinically ineffective, although it forms bifunctional cross-links in the in vitro reaction with DNA1–4. A question still unsolved is why only cis-DDP is an antitumor drug.