C1q (IPR001073) contains a small spherical N-terminal domain, a collagen-rich central Gly/Pro region, and a conserved C-terminal region . The C-terminal globular domain of the C1q subfraction and collagen is required for the triple helix to fold correctly and for protein-protein recognition events to occur . The domain is found in multimeric proteins and EMILIN proteins. It is required for the proper assembly of proteins . The C1q domain's globular head is a trimer . C1q is also a complement receptor 1 binding protein found on the surface of red blood cells.