G Protein-Coupled Receptors (GPCRs) are a large family of cell surface proteins that play a crucial role in transmitting signals from outside the cell to the inside. They respond to various stimuli, such as hormones, neurotransmitters, and environmental signals, activating internal cellular pathways through the binding of G proteins. This process is essential for many physiological functions, including vision, taste, and immune responses. When a ligand, like a hormone, binds to a GPCR, it causes a change in the receptor's shape, which then activates the associated G protein. This activation triggers a cascade of biochemical events within the cell, leading to a specific response. GPCRs are involved in numerous diseases, making them important targets for drug development.