C1q is part of the C1 enzyme complex, which activates the serum complement system. The residues of the globular domain in C1q share homology with several other secreted and membrane-bound collagen or collagen-like proteins, including pre-cerebellin and collagen types VIII and X, as well as the human and mouse genes encoding Apm1/BPB80 and AdipoQ/ACRP30, respectively. These various C1q-related proteins are found in adipose serum, corneal endothelium, chondrocytes and cerebral Purkinje cells. C1qL1 (complement component 1, q subcomponent-like 1), also known as CRF or C1QRF, is a polypeptide with a hydrophobic signal sequence, a collagenous region and a globular domain at the carboxy terminus, which shares homology to the C1q globular domain. C1qL1 transcripts are most abundant in areas of the nervous system that are associated with motor function, including cerebral Purkinje cells, the pons, the accessory olivary nucleus, and the red nucleus. The similarity of mouse C1qL1 to human C1qL1 suggests a conserved and important role for the protein. In humans, the gene encoding C1qL1 maps to chromosome 17q21.