Oxoeicosanoid receptor 1 (OXER1) also known as G-protein coupled receptor 170 (GPR170) is a protein that in humans is encoded by the OXER1 gene located on human chromosome 2p21; it is the principal receptor for the 5-Hydroxyicosatetraenoic acid family of carboxy fatty acid metabolites derived from arachidonic acid. The receptor has also been termed hGPCR48, HGPCR48, and R527 but OXER1 is now its preferred designation. OXER1 is a G protein-coupled receptor (GPCR) that is structurally related to the hydroxy-carboxylic acid (HCA) family of G protein-coupled receptors whose three members are HCA1 (GPR81), HCA2 (Niacin receptor 1), and HCA3 (Niacin receptor 2); OXER1 has 30.3%, 30.7%, and 30.7% amino acid sequence identity with these GPCRs, respectively. It is also related (30.4% amino acid sequence identity) to the recently defined receptor, GPR31, for the hydroxyl-carboxy fatty acid 12-HETE. Species and tissue distribution Orthologs of OXER1 are found in various mammalian species including opossums and several species of fish; however, mice and rats lack a clear ortholog of OXER1. This represents an important hindrance to studies on the function of OXER1 since these two mammalian species are the most common and easiest models for investigating the in vivo functions of receptors in mammals and by extrapolation humans. Since mouse cells make and respond to members of the 5-HETE family of agonists, it is most likely that mice do have a receptor that substitutes for OXER1 by mediating their responses to this agonist family. Recently, A G protein-couple receptor of the hydroxy carboxilic acid subfamily, Niacin receptor 1, has been proposed to mediate the responses of mouse tissues to 5-oxo-ETE. OXER1 is highly expressed by human white blood cells, particularly eosinophils and to a lesser extent neutrophils, basophils, and monocytes; by bronchoalveolar macrophages isolated from human bronchoalveolar lavage washings; and by the human H295R adrenocortical cell line. Various types of human cancer cells lines express OXER1; these include those of the prostate, breast, lung, ovaries, colon, and pancreas. OXER1 is also expressed by the human spleen, lung, liver, and kidney tissues. The exact cell types bearing OXER1 in these tissues has not been defined. A recent study has found that cats express the OXER1 receptor for 5-oxo-ETE, that feline leukocytes, including eosinophils, have been found to synthesize and be very highly responsive to 5-oxo-ETE, and that 5-oxo-ETE is present in the bronchoalveolar lavage fluid from cats with experimentally induced asthma; these findings suggest that