ALZEVIT: Nationwide Digital APOE ε4 Cohort for Early Alzheimer's Disease Prevention and Trial Readiness

Inclusion Criteria: * Participants who sign the electronic Informed Consent form. * Adults aged over 45 years old (inclusive) at the time of consent. * Participants residing in France. * Participants able to read and understand the study information in French or with the help of a legal representative. * Participants are capable of electronically signing the consent form. * Participants affiliated with the French national health insurance system. * Participants with access to the internet and a valid email address. * Participants can self-collect and return a saliva sample using a provided kit (ORAcollect® Dx device). Exclusion Criteria: * Participants who do not sign the Informed Consent form. * Participants who are unable to read or understand French. * Participants younger than 45 years at the time of consent. * Participants with known cognitive impairments, severe dementia, or psychiatric conditions that prevent informed consent. * Participants with a documented substance use disorder (alcohol or drug dependence) within the past 24 months. * Participants who cannot be reliably contacted by email or phone. * Participants previously enrolled in the ALZEVIT study or its pilot phase. * Participants who are legally protected adults or under guardianship, curatorship, or judicial/administrative custody. * Participants whose physical condition does not allow them to perform the tasks required by the study. * Participants who, in the judgment of the Investigator, are unlikely to comply with study procedures. * Participants who are study staff (investigators, sub-investigators, coordinators, assistants) or their immediate family members. * Participants who are incarcerated or deprived of liberty by judicial or administrative decision. * Participants with any disease or condition that, in the opinion of the Investigator, would compromise participation or safety.