How do signals from outside the cell cause a response inside it? Such outside signals could be hormones or neurotransmitters. To notice them, the cell's surface possesses receptors. One of the key classes of such receptors are so-called G protein-coupled receptors (GPCRs). They are proteins placed on the cell's membrane. Once an outside signal activates them, they trigger processes inside the cell with far-reaching impacts on cell growth, migration, and metabolism as well as cell-to-cell communication. This group of more than 800 receptors also plays a vital role in many diseases. Therefore, GPCRs have become important targets of drugs. In fact, 35% of all drugs approved by the US Food and Drug Administration (FDA) target GPCRs, accounting for an annual market of estimated 180 billion US dollars.