Specifically, in a cohort of patients with early PD (n = 288), an increase in met alleles (i.e., lower COMT activity and putatively higher dopamine levels) was associated with impaired performance on the CANTAB one-touch Tower of London, a planning task dependent on prefrontal cortical areas, and the impairment was greatest in patients on dopaminergic medications (75). Here, COMT is linked to Parkinson disease.