Late Onset Alzheimer's Disease

Inclusion Criteria: * Established diagnosis of definite or probable AD or have a diagnosis of a related neurodegenerative disorder such as Frontotemporal Dementia (FTD) or Lewy Body Dementia (LBD) (will also recruit sporadic FTD and LBD) cases. * a living sibling with probable or possible AD; * a third living relative affected with AD (onset age 50 or older) or unaffected (60 or older); * participants in the proband's generation with an identified companion serving as an informant; * participants who have capacity to consent or participants lacking capacity to consent with a surrogate/proxy in place to provide consent. Exclusion Criteria: * failure to identify an appropriate informant; * uncertainty of the clinical diagnosis of Alzheimer's disease or other related disorder; * discovery of additional diagnosis that could account for the clinical manifestations; * unwillingness to participate; * failure to identify a living sibling with AD or other related disorder (except in the cases of sporadic FTD and sporadic LBD); * participants lacking the capacity to consent who do not have a surrogate or proxy or next of kin to provide consent.