A novel strategy for monitoring the location of a transgene in a mammal is disclosed. A sodium iodide symporter is genetically fused to either the N-terminus or C-terminus of the product of a transgene through a linker peptide which bears the recognition sequence of a host cell protease. Expression of the transgene confers the activity of the sodium iodide symporter (NIS)to a host cell which expresses the transgene. Subsequent administration of labeled iodine results in transport of the labeled iodine into the cell bearing the NIS, which can then be localized and measured using standard imaging techniques. The system is particularly useful for monitoring the location of therapeutic transgenes and tissue-specific distribution of the therapeutic gene product.