GPR68 and cancer: A decrease in extracellular pH strongly correlates with EMT and promotes cancer cell proliferation, migration, invasion, and metastasis in acidic environments through the pH-sensing G protein-coupled receptors (GPCRs) GPR4, GPR65 (T-cell death-associated gene 8 protein, TDAG8), GPR68 (ovarian cancer G-protein coupled receptor 1, OGR1), and GPR132 (G2 accumulation protein, G2A) [59].