Bioluminescence Animal Imaging. All animal handling was performed in accordance with Stanford University's Animal Research Committee guidelines. HT1080 cells constitutively overexpressing a BRET fusion protein, RLuc8 or RLuc8.6, were trypsinized, pelleted, and resuspended in PBS at 3 × 106 cells in 150 μL. The PBS cell suspension was injected through the tail vein in a set of anesthetized (2% isoflurane oxygen) female nude mice (Nu/Nu; Charles River). After cell injection, mice were removed from the anesthesia. CLZ or CLZ-v (35 μg/mouse) diluted in PBS (150 μL total volume) was injected through the tail vein 1.5 h after cell suspension injection. Mice were imaged immediately using an IVIS-200 or IVIS-Spectrum equipped with a charge-coupled device camera. Imaging was performed in sequence luminescence scan mode using either open or 20-nm bandwidth spectral filters appropriate for each donor/acceptor system, with 1 min acquisition time at each filter. Regions of interest were drawn over the mice lungs area for each image. The signal from control mice injected only with the luciferase substrate was subtracted from the total signal. All data were analyzed using Living Image 3.1 software. For experiments using various cell numbers of reporter cells, imaging was preformed as described above. Although the reporter cell number was varied, the total number of cells injected in mice was kept constant (3 × 106) by adding regular HT1080 cells. Detailed procedures for imaging the rapamycin-induced FRB/FKBP12 interaction in mice are given in SI Materials and Methods.