C7 is a single-chain plasma glycoprotein involved in the cytolytic phase of complement activation (Hobart et al., 1995). Mutations in the C7 gene cause C7 deficiency, a defect associated with increased susceptibility to neisserial recurrent infections (Barroso et al., 2004). Hobart et al. (1978) identified 3 structural forms of C7, concluded they are the products of 3 codominant alleles at an autosomal locus, and found that the C6 and C7 loci are closely linked to each other but not to the HLA complex. By Southern blot analysis of hybrid cell DNAs using cDNA probes, Jeremiah et al. demonstrated that the human C6 and C7 genes are located on chromosome 5. See Coto et al. (1991) for linkage information indicating that C6, C7, and C9 are closely situated; C9 was mapped to 5p13.