A family of proton-sensing G-protein-coupled receptors (GPCRs), ovarian cancer G-protein-coupled receptor 1 (OGR1, also known as GPR68), G-protein-coupled receptor 4 (GPR4), and T-cell death-associated gene 8 (TDAG8, also known as GPR65), is activated by protons by differing degrees of sensitivity to pH [14]. This evidence concerns the gene GPR4 and ovarian cancer.