A reduction in the level of α4 nAChR subunit protein in the brains of 12‐month‐old APP/PS2 mice is associated with clear cognitive impairment and a decrease in the accumulation of [125I]5IA (an imaging probe for α4β2 nAChR), indicating that such a decrease might serve as a biomarker for cognitive impairment associated with AD [98]. This evidence concerns the gene APP and Alzheimer disease.