Additional studies have shown that extracellular protons can be recognized by a subfamily of G protein-coupled receptors, which includes four receptors: G protein-coupled receptor 4 (GPR4), ovarian cancer G protein-coupled receptor 1 (OGR1), T cell death-associated gene 8 (TDAG8), and G2 accumulation (G2A). This evidence concerns the gene GPR68 and ovarian cancer.