GPR4 and ovarian carcinoma: Shortly after the characterization of ovarian cancer G-protein-coupled receptor 1 (ORG1) and G-protein-coupled receptor 4 (GPR4) as high affinity SPC receptors, evidence was published showing that both of these receptors act as proton-sensing G-protein-coupled receptors (GPCRs) in response to extracellular pH change [22].