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Up-regulation of mu-opioid receptors (MOR) in certain tumors has been reported. Understanding which tumors have up-regulation of MOR could prove helpful in this field of research if indeed opioids have growth effects on certain tumor cells. We attempt to create a Mu-score system that would quantify the mu receptor density of various tumors. Our study investigates immunohistochemical analysis of various tumor tissues with two commercially available opioid antibodies, both of which showed patchy reactivity within tumor tissue and nonspecific background staining. These findings suggest that currently available immunohistochemical stains are of limited utility both in assessing tumor tissue MOR density and in creating the proposed Mu-score system.