Importantly, individuals with schizophrenia who carry the Val(158)Met COMT mutation have a reduction in odor identification accuracy, and TH is significantly increased in people with COMT-Val or COMT-Met mutations, considered to be a cellular response to suboptimal COMT activity—potentially explaining the increased levels of TH in the PD olfactory bulb [2, 35]. This evidence concerns the gene COMT and Parkinson disease.