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coding the human or species specific NIS genes. NIS function in transduced MSC was first validated in vitro; NIS expressing MSC (MSC_NIS) from multiple species concentrated high levels of I-125 with no side effects. The sensitivity of cell detection was determined by transplanting a known number of MSC_NIS subcutaneously into mice. We can reliably detect 2x105 MSC_NIS in mice using the newly acquired U-SPECT II machine. Canine MSC derived from the bone marrow were surprisingly robust; viable cells were still detected (albeit lower numbers) at day 28 in the athymic mice. In contrast, NIS signals from adipose tissue derived rat or human MSC disappeared by day 7 post transplantation. Using PET imaging and F18-TFB, sensitivity of imaging could be significantly improved to detect lower numbers of cells. We are currently evaluating the use of NIS for tracking natural killer cells and transplanted hemapoietic stem cells and results will be presented.