GPR4 and ovarian cancer: Ovarian cancer G protein-coupled receptor 1 (OGR1), T cell death-associated gene 8 (TDAG8), G protein coupled receptor 4 (GPR4) and G2 accumulation (G2A) are all belong to proton-sensing GPCRs family, they share 40–50% homology with different action modes and signaling pathways19.