• 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.