gC1qR is present in plasma and the extracellular matrix. The molecule is an unusually acidic, single chain protein with an apparent molecular weight of 33 kDa. It does not possess a conventional sequence motif compatible with a membrane spanning segment nor a consensus site for a GPI anchor. gC1qR migrates as a single chain under both reducing and non-reducing conditions, but it behaves as an oligomer on gel-filtration in non-dissociating conditions. Its multimer formation may be a critical process by which the gC1qR molecule increases its affinity for multivalent ligands such as C1q. gC1qR has been shown to inhibit complement activation by preventing the binding of C1q to antibodies, suggesting that the binding site for gC1qR and the binding site for immune complexes, which are present on the C1q globular ‘heads’, may be located at the same position. gC1qR is capable of interacting with several proteins involved in blood clotting, namely, thrombin, prothrombin, the heparin binding form of vitronectin, the ternary complex, vitronectin-thrombin-antithrombin, as well as high-molecular-weight kininogen and Hageman factor. Besides its role in the complement pathway, gC1qR participates in enhancement of Fc-receptor and CR1-mediated phagocytosis, procoagulant activity on platelets, and chemotactic activity on mast cells, eosinophils, neutrophils, and fibroblasts. gC1qR is expressed on a wide variety of cells and can serve as a binding site for plasma and microbial proteins. Its antigenic sites may be cryptic on cells in suspension but become exposed when the cells are activated or fixed by bifunctional cross-linkers. Probably it is also expressed on the cell membrane as a trimer. Crosslinking or activation may thus bring about a trimeric assembly of gC1qR followed by a conformational change within the molecule, thereby exposing epitopes which are otherwise hidden. A form of gC1qR is also found inside the cell. Intracellular gC1qR has been shown to bind the cytoplasmic tail of the α1B-adrenergic receptor and to PKCμ.