G-protein coupled receptors (GPCRs) are a massive family of proteins found on the surface of cells, acting as the primary gatekeepers of cellular communication. When a specific molecule, such as a hormone or neurotransmitter, binds to the exterior portion of the GPCR, the receptor undergoes a conformational change. This change allows it to activate a G-protein inside the cell, which then triggers a cascade of internal signals leading to a physiological response, such as a change in heart rate or the sensation of light. Because GPCRs are involved in almost every physiological process, they are the target of approximately one-third of all modern pharmaceuticals. However, drugs that target GPCRs often cause side effects because a single type of receptor might be located in multiple organs, leading to unintended reactions in tissues other than the one being treated.