Depression can precede dementia, or at least depressed patients are at greater risk of developing PDD.115 In a recent study, neocortical cholinergic innervation was assessed in 12 non-demented PD patients, 6 PDD, and 10 normal controls116 by means of dynamic PET scanning of previously injected [11C]methyl-4-piperidinyl propionate radioligand, a selective substrate for AChE hydrolysis. The gene discussed is ACHE; the disease is major depressive disorder.