Enroll-HD is a longitudinal, observational, multinational study that integrates two former Huntington's disease (HD) registries-REGISTRY in Europe, and COHORT in North America and Australasia-while also expanding to include sites in Latin America. More than 30,000 participants have now enrolled into the study. With annual assessments and no end date, Enroll-HD has built a large and rich database of longitudinal clinical data and biospecimens that form the basis for studies developing tools and biomarkers for progression and prognosis, identifying clinically-relevant phenotypic characteristics, and establishing clearly defined endpoints for interventional studies. Periodic cuts of the database are now available to any interested researcher to use in their research - visit www.enroll-hd.org/for-researchers/access-data/ to learn more.

The primary objective of Enroll-HD is to develop a comprehensive repository of prospective and systematically collected clinical research data (demography, clinical features, family history, genetic characteristics) and biological specimens (blood) from individuals with manifest HD, unaffected individuals known to carry the HD mutation or at risk of carrying the HD mutation, and control research participants (e.g., spouses, siblings or offspring of HD mutation carriers known not to carry the HD mutation). Enroll-HD is conceived as a broad-based and long-term project to maximize the efficiencies of non-clinical research and participation in clinical research. With more than 150 active clinical sites in 23 countries, Enroll-HD is now the largest HD database available and is accessible to any interested researcher - visit www.enroll-hd.org/for-researchers/access-data/ to learn more.

Inclusion Criteria:

• Carriers: This group comprises the primary study population and consists of individuals who carry the HD gene expansion mutation.
• Controls: This group comprises the comparator study population and consists of individuals who do not carry the HD expansion mutation.

These two major categories can be further subdivided into six different subgroups of eligible individuals:

• Manifest/Motor-manifest HD: Carriers with clinical features that are regarded in the opinion of the investigator as diagnostic of HD.
• Pre-Manifest/-Motor-manifest HD: Carriers without clinical features regarded as diagnostic of HD.
• Genotype Unknown: This group includes a first or second degree relative (i.e., related by blood to a carrier) who has not undergone predictive testing for HD and therefore has an undetermined carrier status.
• Genotype Negative: This group includes a first or second degree relative (i.e., related by blood to a carrier) who has undergone predictive testing for HD and is known not to carry the HD expansion mutation.
• Family Control: Family members or individuals not related by blood to carriers (e.g., spouses, partners, caregivers).
• Community Controls: Individuals unrelated to HD carriers who did not grow up in a family affected by HD. Data collected from community controls will be used for generation of normative data for sub-studies.

Participant status will be captured in the study database using 2 variables: 1) Investigator Determined Status: this will be based on clinical signs and symptoms and genotyping performed as part of medical care, and will be updated at every visit; and 2) Research Genotyping Status: this will be based on genotyping conducted as part of Enroll-HD study procedures. Based on research genotyping, participants will be reclassified under this variable from Genotype Unknown to 'Carriers' or 'Controls'. Investigators and participants will be blinded to this reclassification.

Exclusion Criteria:

• Individuals who do not meet inclusion criteria,
• Individuals with choreic movement disorders in the context of a negative test for the HD gene mutation.
• For Community Controls: those individuals with a major central nervous system disorder will be excluded (e.g. stroke, Parkinson's disease, multiple sclerosis, etc.).

Participants under 18 may be eligible to participate (if they have juvenile-onset HD).

Patients with HD and their family members are recruited from specialty clinics (Human Genetics, Neurology, Psychiatry) that advise and treat people affected by HD. In addition, in some areas community clinics and neurologists who see HD patients recruit participants for this study. Participants also receive information about the study through websites, clinical practices, support groups, advocacy newsletters, etc. and place a direct request to be considered for participation in the study. Community controls are identified by study site staff (using advertisements, flyers and newsletters) with the support of the Enroll-HD operational staff.