(a) A schematic diagram showing the behavior of the PTD-ODD-Luciferase-NIRF dye (POL-N) imaging probe in cells with (+) or without (?) HIF activity. CTZ, luciferase substrate coelenterazine. (b) Detection of subcutaneous tumors in mice using bioluminescence imaging with fire fly luciferase (HRE-Fluc), BRET probe imaging, and fluorescence imaging (FI) of the same mouse systemically injected with POL-N. Yellow arrowheads in FI indicate fluorescence signals from the kidneys. The image visualized using the BRET probe almost perfectly matches that of HIF-active tumor cells visualized using the HRE-Fluc reporter gene inserted into the tumor cell genome. (c) Tumor/muscle and tumor/kidney ratios of signals detected by BRET probe imaging and FI.