The largest genome-wide genetic association study (GWAS) to date identified multiple loci linked to the immune system among the strongest associations in the over 100 loci associated with schizophrenia.127 Although the potential impact of many of these has yet to be determined, one locus identified implicated the complement component 4 (C4).128 Alleles of this gene were subsequently shown by Sekar et al.128 to associate with schizophrenia in proportion to the amount of C4A that they generate, and greater expression of C4 in brains of individuals with schizophrenia was related to genotype. The gene discussed is C4A; the disease is schizophrenia.