Complement Component 1, q subcomponent (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. Southern blot analysis of chromosomal DNA from vertebrate species demonstrated highest similarity between the C1qB genes, followed by C1qC and finally C1qA. Sequence comparison of C1q from three different species have shown that the B chains have the strongest similarity. C1q was already present at embryonic day 14 (E14) and showed little change in abundance through six weeks postnatal. At E16, C1qB mRNA was present at high abundance in putative microglia/macrophages in cortical marginal and intermediate zones, and hippocampal analge.