EBI2, GPR18 and GPR17--three structurally related, but biologically distinct 7TM receptors

7TM receptors constitute one of the largest superfamilies of proteins in the human genome. They are involved in a large number of physiological and pathological processes in the human body and thus represent major and important drug targets for the pharmaceutical industry. Although the majority have been deorphanized, many remain orphan, and these orphan receptors constitute a large pool of potential drug targets. This review focuses on one of these orphan targets, the Epstein-Barr Virus-induced receptor 2, EBI2 (or GPR183), together with two structurally related receptors, GPR17 and GPR18. The pharmacology and "druggability" of these three receptors is reviewed through a thorough description of their structural and functional properties and in vivo biology together with a status of currently available ligands for these receptors.