In C3H/Hej mice, changes in substance P and calcitonin gene–related peptide are observed in models of inflammatory (CFA) and neuropathic (sciatic nerve transection or L5 spinal nerve ligation) pain models, with no changes in levels of substance P and calcitonin gene–related peptide observed in the model of cancer pain (injection of osteolytic sarcoma cells into the femur), suggesting that cancer induces a unique, persistent pain state (54). This evidence concerns the gene TAC1 and cancer.