The most common type of receptor on cell surfaces — activated by peptide hormones, neurotransmitters, and many drugs, triggering changes inside the cell via a relay protein.The largest receptor superfamily in the human genome (~800 members), characterized by seven transmembrane domains. G protein coupling determines what happens inside the cell after activation: Gs activates adenylyl cyclase, producing cAMP; Gi inhibits adenylyl cyclase; Gq activates phospholipase C, producing IP3 and DAG. The vast majority of peptide hormone receptors are GPCRs. Understanding which G protein a receptor couples to predicts the downstream cellular response.