Post Schizophrenia Risk Increases When Small Genetic Differences Combine
(Psych Central News) When combined, single DNA letter difference in two separate genes may increase one’s risk for developing schizophrenia, according to Johns Hopkins researchers. Scientists have had a difficult time pinpointing the causes for psychiatric diseases such as schizophrenia and autism, because these disorders may be triggered by several small genetic changes that alone may not trigger

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IMHO I think MS is like that.