We found the LOD for NIS-GFP+ Tregs in our instrument to be ∼104 cells/million (Figure S4), roughly one order of magnitude lower than what we found previously for cancer cells;27 we attributed this to the Tregs being about ∼2-fold smaller in diameter (amounting to ∼1⁄8 in volume, taking up at least 8-fold less radiotracer per cell). This evidence concerns the gene SLC5A5 and cancer.