We demonstrate a novel method of concentrating radiation for tumor imaging or killing. The rat sodium/iodide symporter gene (rNIS) was cloned into a retroviral vector for transfer into cancer cells to mimic the iodide uptake of thyroid follicular cells. In vitro iodide transport shows that the symporter functions similarly in rNIS-transduced tumor cells and rat thyroid follicular cells. rNIS-transduced and control nontransduced (NV) human A375 melanoma xenografts established in vivo in athymic nude mice were imaged using a gamma camera after i.p. injections of 123I. The rNIS-transduced human A375 melanoma tumors are visually distinguishable from and accumulate significantly more radionuclides than NV tumors. In vitro clonogenic assays confirm efficacy and clearly show that rNIS-transduced A375 human melanoma, BNL.1 ME murine transformed liver, CT26 murine colon carcinoma, and IGROV human ovarian carcinoma can be selectively killed by the induced accumulation of 131I. Thus, NIS-based gene therapy may have both diagnostic and therapeutic applications for cancer.