OPRM1 and neuroblastoma: In contrast, in SH-SY5Y human neuroblastoma cells that endogenously express RGS4 and MOR, knockdown of RGS4 did not affect responses to the MOR agonist morphine (Wang et al., 2009), suggesting that the ability of RGS4 to regulate MOR may be determined by the cell type and/or the agonist.