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Catechol-O-Methyltransferase val158met effect may be due to human optimism.
Hall KT, Lembo AJ, Kirsch I, Ziogas DC, Douaiher J, et al. Polymorphism Predicts Placebo Effect in Irritable Bowel Syndrome. PLoS ONE 7(10): e48135, Published: October 23, 2012, Reader Comment.