Accordingly, if one uses such a polyclonal population of NIS-transduced cells in a similar study, it would be speculated that a small population of cancer cells not transduced with a reporter gene will gradually increase in the primary tumor and may dominantly form metastases, since non-transduced cells are generally faster in tumor growth and metastatic formation compared with transduced cells as shown in our study as well. This evidence concerns the gene SLC5A5 and cancer.