Human solid tumors contain hypoxic regions that have considerably lower oxygen tension than normal tissues. They are refractory to radiotherapy and anticancer chemotherapy. Although more than half a century has passed since it was suggested that tumour hypoxia correlates with poor treatment outcomes and contributes to recurrence of cancer, no fundamental solution to this problem has been found. Hypoxia-inducible factor-1(HIF-1) is the main transcription factor that regulates the cellular response to hypoxia. It induces various genes, whose function is strongly associated with malignant alteration of the entire tumour. The cellular changes induced by HIF-1 are extremely important therapeutic targets of cancer therapy, particularly in therapy against refractory cancers. Therefore, targeting strategies to overcome the HIF-1-active microenvironment are important for cancer therapy. To Target HIF-1-active/ hypoxic tumor cells, we developed a fusion protein drug, PTD-ODD-Procaspase-3 that selectively induces cell death in HIF-1-active/hypoxic cells. The drug consists of the following three functional domains: the protein transduction domain (PTD), which efficiently delivers the fusion protein to hypoxic tumor cells, the ODD domain, which has a VHL-mediated protein destruction motif of human HIF-1α protein and confers hypoxia-dependent stabilization to the fusion proteins, and the human procaspase-3 proenzyme responsible for the cytocidal activity of the protein drug. In vivo imaging systems capable of monitoring HIF-1 activity in transplanted human cancer cells in mice are useful in evaluating the efficiency of these drugs and in study of HIF-1-active tumor cells.