,22 One of the most promising host reporter genes is the human sodium iodide symporter (NIS), which has been shown to be well tolerated in different cell types, including cancer cells, cardiomyocytes, stem cells, and T cells.27, 28, 29, 30 In a proof-of-principle study employing retroviral transduction methodology, we demonstrated ex vivo engineering of murine Tregs to express NIS and detected the cells in vivo 24 h after administration by SPECT imaging.31 Here, SLC5A5 is linked to cancer.