Study of Tau Imaging With the Use of [18F]MK-6240 Tracer

Inclusion Criteria: 1. Participants have met all eligibility criteria for enrollment into the Dominantly Inherited Alzheimer's Network (DIAN) and DIAN Extended Registry (DIAN EXR) study enrollment criteria. 2. Male or female participants, at least 18 years of age. 3. Cognitively normal, or with mild dementia, as assessed clinically. 4. Participant is able and willing to undergo testing (magnetic resonance imaging (MRI)or computed tomography (CT), PET, radioactive tracer injection; forthose unable to undergo MRI, CT will be used to generate regions-of interest). 5. Pre-menopausal women will undergo a urine pregnancy test within 24 hours of drug administration. - Exclusion Criteria: 1. Has any condition that, in the Investigator's opinion, couldincrease risk to the participant, limit the participant's ability to tolerate the experimental procedures, or interfere with the collection/analysis of the data (for example, participants with severechronic back pain might not be able to lie still duringthe scanning procedures). 2. Is deemed likely unable to perform the imaging procedures for any reason. 3. Has hypersensitivity to \[18F\]MK-6240or any of its excipients. 4. Contraindications to PET, PET-CT or MR(e.g. electronic medical devices, inability to lie still for long periods) that make it unsafe for the individual to participate. 5. Severe claustrophobia. 6. Women who are currently pregnant or breast-feeding, and women who do not agree to use reliable contraception, or to refrain from sexual activity for 24 hours following administration of the \[18F\]MK-6240injection will be excluded from the study. 7. Currently participating in any research studyreceiving an active study medication for AD, an investigational drug, device, imaging, or placebo within the past 30 days before screening, and throughout this clinical trial up to 2-weeks past any study-related procedures. 8. Other than the DIAN study, current or recent (within 12 months prior to screening) participation in research studies involving radioactive agents such that the total research-related radiation dose to the participant in any given year would exceed the limits set forth in the US Code of Federal Regulations (CFR) Title 21 Section 631.1. http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?FR=361.1. It is the responsibility of each site to confirm the date of the most recent PET scan and to work within the guidelines of the local Radioactive Drug Research Committee (RDRC) regarding the imaging interval. -