C1q has a hexameric structure with six triple helices being formed between the collagen-like sequences of its 6A, 6B and 6C chains and globular 'heads' being formed from the C-terminal portions of these chains. Thus the molecule is composed of six globular 'heads' linked via six collagen-like 'stalks' to a fibril-like central region. The collagen-like regions interact with the C1r2-C1s2 proenzyme complex to yield C1, the first component of complement. Activation of C1 by immune complexes is mediated by the ionic binding of two, or more, of the 'heads' of C1q to the C gamma 2 domains of IgG or C mu 3 domains of IgM. Removal of the activated C1r2-C1s2 complex by C1-inhibitor leaves the collagen-like 'stalks' of C1q free to bind to cell-surface C1q-receptors. Three other proteins, conglutinin, mannan-binding protein and lung-surfactant protein are structurally similar in many respects to C1q and may also bind to receptors via their collagen-like regions although they show carbohydrate binding properties via their globular 'heads'. The availability of cDNA and genomic clones for the chains of C1q has allowed studies on their location and organisation and also on the analysis of DNA from C1q deficient individuals. The human A and B chain genes are located approx. 20kb apart on chromosome 1p, each gene being approx. 2kb long and each containing one intron of about 1kb. Genetic C1q deficiency in one individual has been shown to be due to the generation of a stop codon by a single point mutation at residue 150 in the coding region of the B chain.