These investigations have in common that PD patients received no other PD drug overnight, only 100 mg levodopa/25 mg carbidopa with and without COMT-inhibitor in the currently approved and thus applied dosing (200 mg entacapone with each levodopa dose, or tolcapone 100 mg t.i.d. or 50 mg opicapone o.i.d) and showed a positive motor response to the levodopa administration during the investigation interval. Here, COMT is linked to Parkinson disease.