A recent study (Sekar et al., 2016) found that the association between schizophrenia and the MHC locus to a considerable extent stems from many common, structurally distinct alleles of the complement component 4 (C4), and these alleles were moreover found to affect the expression of C4A and C4B in the brain and to be associated with schizophrenia in proportion to their effect on C4A expression. The gene discussed is C4A; the disease is schizophrenia.