Bioluminescence imaging (BLI) of orthotopic tumors. Human primary glioblastoma multiforme cells, engineered prostate cancer cells, and multiple myeloma cells were engineered to express firefly luciferase. Orthotopic tumors were established by stereotactic implant into the brain, injection into the ventral prostate, or intravenous injection, respectively. Tumor burden was assessed by serial bioluminescence imaging. For each model, all images are represented at the same scale.