G protein-coupled receptors (GPCRs) form a large family of receptors characterized by their seven transmembrane domains. Ligands bind at the extracellular part and activate signal transduction pathways inside the cell which evoke cellular responses. GPCRs are activated by different kinds of ligands varying largely in MW including hormones and neurotransmitters. The human genome contains about 400 GPCRs. GPCRs are involved in many diseases and thus are important drug targets (around half of all modern medicinal drugs!).