Interestingly, a randomized controlled trial demonstrated that donepezil treatment was associated with lower occurrence of psychosis only in PD patients not carrying the APOE ε4 allele [185], suggesting that APOE genotype may at least partially account for differences in the clinical response to acetylcholinesterase inhibitors among PD patients, in regard to psychotic symptoms. The gene discussed is APOE; the disease is Parkinson disease.