Olfactory receptors are difficult to functionally express in heterologous cells. They are typically retained in the endoplasmic reticulum of cells commonly used for functional expression studies and are only released to the plasma membrane in mature cells of the olfactory receptor neuron lineage. ODR-4, a protein that is necessary for plasma membrane trafficking of a chemosensory receptor in nematodes, facilitates trafficking of rat olfactory receptor U131 in odora and Chinese hamster ovary cells. ODR-4 is a conserved tail-anchored transmembrane protein. ODR-4 localizes predominantly to the ER in vivo, and the soluble ODR-8 UfSP2 protein also shows some ER localization.