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. A deficiency in 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 C-chain polypeptide of human complement subcomponent C1q. Alternatively spliced transcript variants that encode the same protein have been found for this gene. [provided by RefSeq, Jul 2008]