Inclusion Criteria:  Aged 55-75. Written informed consent obtained from participant or legal guardian prior to any study-related procedures. The diagnosis of AD is made using the National Institute on Aging and the Alzheimer's Association (NIA-AA) criteria. As for non-AD dementia, the McKeith criteria are used for DLB,the revised diagnostic criteria proposed by the International behavioral variant (bvFTD) Criteria Consortium for bvFTD,the Gorno-Tempini criteria for the semantic variant FTD or non-fluent aphasia, the Movement Disorder Society Task Force criteria for PDD, the vascular behavioral and cognitive disorders (Vas-Cog) criteria for VaD, the Armstrong's criteria for CBD, the CDC's diagnostic criteria for CJD, etc. In addition, normal cognition is supported by MMSE, CDR and other cognitive function scales.  Exclusion Criteria:  Other medical or psychiatric illness. No one can serve as an informant. Refused to complete a cognitive test and provide biospecimen.