Genetic testing of patients with PD has value for both appreciating the relevance of genes known to be associated with PD as well as those relevant to treatment [14] Catechol-O-methyltransferase (COMT) polymorphisms may alter the response to PD drugs since COMT inhibition is required for optimal benefit from carbidopa–levodopa [15]. Here, COMT is linked to Parkinson disease.