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More than 100 genetic variants have been associated with schizophrenia risk in genome-wide association studies, including areas near C4, a complement component gene in the major histocompatibility complex (which is involved in immunity) on chromosome 6. Because it was unknown how C4 polymorphisms were related to the increased risk, researchers conducted a series of experiments.
In postmortem brain samples, C4A and C4B (the two CA genes found in humans) were found to vary by structure, copy number, and length (depending on the presence of a human retrovirus). In a study of family trios (mothers, fathers, and offspring), 90% had four common structural forms — various combinations of various lengths of C4A and C4B. In an analysis of single nucl…