The recent development of a PET tracer,[18F]MK-6240(an[18F]tau imaging agent,CerveauTechnologies)that has high affinity for the human phosphorylated tau deposits in AD brain offers new opportunities to investigate tau pathology. The investigators will evaluate this imaging agent in individuals from families with a known Autosomal Dominant Alzheimer's Disease (ADAD) mutation. This study of tau PET using [18F]MK-6240 is performed in conjunction with DIAN and DIAN Extended Registry (DIAN-EXR).

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. -

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.

De-identified study data will be maintained at Washington University. This de-identified data may also be shared with other investigators doing research in similar fields such as Alzheimer disease, other neurological disorders or related brain diseases andabnormalities. These investigators may be at Washington University or at other research institutions. This de-identified data may also be shared with large repositories for broad sharing with the research community. If the participant's research data is placed in one of these repositories, only qualified researchers who have received prior approval from individuals that monitor the use of the datawill be able to look at this information. Information from this research study has been or will be entered into a clinical trial databank that is maintained by the National Institutes of Health/National Library of Medicine.