G protein-coupled receptors (GPCRs) are a large family of seven transmembrane protein receptors found in eukaryotes. Coupling with G proteins, these receptors are activated by binding to diverse extracellular ligands including light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, etc. The activated GPCRs further induce intracellular signal transduction pathways and, ultimately, cellular responses. GPCRs are involved in a wide variety of physiological processes and many diseases. To date, many medicinal drugs are GPCR-targeting, and there is an increasing interest in the study of discovering novel GPCR-related drug candidates.