Proton-sensing GPCRs—notably GPR4, GPR65 (also known as T-cell death-associated gene 8, TDAG8), and GPR68 (also known as ovarian cancer G-protein-coupled receptor 1, OGR1)—are transmembrane receptors that detect extracellular acidification [77,78]. The gene discussed is GPR65; the disease is ovarian cancer.