G protein-coupled receptors (GPCRs) are a large family of proteins found in the cell membrane. They play a crucial role in transmitting signals from outside the cell to the inside, allowing cells to respond to various stimuli, such as hormones, neurotransmitters, and environmental signals. When a molecule binds to a GPCR, it activates an associated G protein, which then triggers a series of cellular responses. These receptors are involved in many physiological processes, including vision, taste, and immune responses. GPCRs are also important drug targets, as they are implicated in numerous diseases, making them a focus for pharmaceutical research and development.