This gene encodes a major constituent of the human complement subcomponent C1q. C1q associates with C1r and C1s in order to yield the first component of the serum complement system. Deficiency of C1q has been associated with lupus erythematosus and glomerulonephritis. C1q is composed of 18 polypeptide chains: six A-chains, six B-chains, and six C-chains. Each chain contains a collagen-like region located near the N terminus and a C-terminal globular region. The A-, B-, and C-chains are arranged in the order A-C-B on chromosome 1. This gene encodes the B-chain polypeptide of human complement subcomponent C1q [provided by RefSeq]. CCDS Note: This CCDS representation uses the 5'-most in-frame start codon, which may be restricted to higher primate species. Two alternative and consecutive downstream start codons, the second of which is most widely conserved, also exist. It is possible that leaky scanning by ribosomes would allow either of the downstream start codons to be used, at least some of the time. The use of a downstream start codon would result in a protein that is 1 aa or 2 aa shorter at the N-terminus. All resulting proteins are predicted to have N-terminal signal peptides, as indicated by SignalP 3.0. There is no experimental evidence showing which start codon is preferentially used in vivo. Publication Note: This RefSeq record includes a subset of the publications that are available for this gene. Please see the Gene record to access additional publications.