G protein receptors, also called GPRs, GPCRs, and seven-transmembrane domain receptors, are a large family of receptors with possibly over 800 members. These receptors sense stimuli outside, convey the signal across the cell membrane, and activate internal signal transduction pathways and cellular responses. GPR77 is involved in the complement system of the innate immune response and may signal through the beta-arrestin pathway, and possibly act as a decoy receptor. It is co-expressed with the C5a receptor on neutrophils and may modulate C5a1 activity.