CCK and early-onset autosomal dominant Alzheimer disease: Multiple studies have explored the link between genetic polymorphisms (e.g., dopamine, ApoE, cholecystokinin [CCK], HOMER, tau, and COMT), and hallucinations in psychiatric disorders typical in younger (e.g., schizophrenia, bipolar disorder) and older (Alzheimer's disease, Parkinson's disease) adults.