It may be useful to develop prodrugs that are selectively activated by oxidative stress in cancer cells to release cell-killing reactive intermediates. However, relatively few chemical strategies exist for the activation of prodrugs under conditions of oxidative stress. Here we provide evidence for a novel process in which oxidation of a thiol residue in the natural product leinamycin E1 by HO and other byproducts of cellular oxidative stress initiates generation of an episulfonium ion that selectively alkylates guanine residues in duplex DNA.