GPR20 is an Orphan-A GPCR with an unknown ligand. It was identified based on sequence similarity with opioid and somatostatin-related receptor genes. GPR20 is active in the absence of ligand, leading to continuous activation of its coupled G proteins. Its expression results in the activation of G(i/o) proteins. The receptor may be involved in cellular processes, including control of intracellular cAMP levels and mitogenic signaling. References: The Journal of Biological Chemistry283, 12747-12755, 2008