It is also possible that the varying roles of CB1 and CB2 receptors may be related to differing experimental factors in the present versus prior studies, including species (rats versus mice), the type of chronic pain model (CCI versus other forms of nerve injury, streptozotocin-induced diabetes, chemotherapy drugs and bone cancer) and even the pain assays used (mechanical/cold allodynia versus spontaneous pain and thermal hyperalgesia). This evidence concerns the gene CNR1 and bone cancer.