G protein coupled receptors (GPCRs) are proteins that are primarily located in a cell’s outer membrane and respond to signals from the surrounding environment to cause a specific biological response inside the cell. These responses constitute a wide range of biologically important and medically relevant processes, including vision, smell, mood regulation, and inflammation. Because GPCRs are involved in many different aspects of human health and disease, they are the targets of almost half of today’s pharmaceuticals. However, many drugs that are supposed to target a specific GPCR produce unwanted side effects, due to interaction with other, closely related GPCRs. One goal of the Common Fund’s Structural Biology program is to determine the exact shape, or structure, of membrane proteins such as GPCRs. This knowledge can then be used to design more selective drugs with the perfect “fit” for their target, much like a specifically designed key fits inside a particular lock.