OPRM1 and bone neoplasm: In the bone cancer pain model, the levels of MOR in pain-related parts, such as the spinal cord, dorsal root ganglia and periaqueductal gray matter, declined [40, 41], probably because of the sustained release of endogenous opioid peptides in the brain, which results in the activation, phosphorylation, and dysregulation of endogenous production of these molecules and therefore a decrease in MOR levels [42].