Later Stage HD Assessments (LSA) is an observational, multinational study aiming at developing two assessments that can be used to measure critical milestones and events during the later stages of Huntington's disease (HD). An important aspect of the evaluation will be to assess whether the assessments can be administered to a companion either in-person or remotely (i.e. by phone contact with the companion). Therefore, these assessments will be evaluated for their internal consistency, reliability and validity. Once established, these assessments may be incorporated into a large scale, global observational study of HD and/or other HD clinical studies as well as use them for planning clinical trials.

The primary objective of this study is to evaluate the internal consistency, reliability and validity of two assessments: (1) UHDRS Structured Interview of Function (SIF) that incorporates the UHDRS Total Functional Capacity, Functional Assessment Scale, and Independence Scale (UHDRS TFC, FAS and IS), and (2) the HD Clinical Status Questionnaire (HDCSQ). There will be two study parts: Part 1 will measure the performance of the UHDRS SIF assessment compared to the original UHDRS TFC, FAS and IS using a cross-over design by collecting data from the Manifest HD gene expansion carrier participants (Manifest HDGEC Participants) and their companions (Companion Participants) via an in-person baseline visit and a follow-up visit by phone. Part 2 will assess the UHDRS SIF and the HDCSQ using a cross-sectional design by collecting data from the Companion Participants via a follow-up visit by phone. Participants of Part 1 are eligible to participate in Part 2.

Inclusion Criteria:

Participants must meet all the following inclusion criteria to participate in this study:

All Participants

1. Individual of either gender
2. Fluent in English
3. Must be willing and able to provide informed consent or have a legal representative willing and able to provide assent

Manifest HDGEC Participants

1. Must be an active participant in the Enroll-HD study. An active participant is defined as a participant who has successfully enrolled in the Enroll-HD study and completed the Enroll-HD study baseline visit.
2. Age≥20 years
3. CAG repeat length≥36
4. DCL=4
5. Inclusion score of 16 at the time of the participant's most recent Enroll-HD study visit (for inclusion score calculation, see Appendix A - Inclusion Score Calculation) Companion Participants

1. Age≥18 years 2. A person who, in his/her opinion, has sufficient interface and knowledge of the Manifest HDGEC Participant's capabilities and daily activities 3. Acceptable to the Manifest HDGEC Participant and the Site Investigator or the Site Investigator's designees

Exclusion Criteria:

People who meet the following criteria will be excluded from participating in this study:

1. Individuals with choreic movement disorders in the context of a laboratory verified non-expansion mutation for the Huntingtin gene
2. Manifest HDGEC Participants with Juvenile-onset HD (rater estimate of age of onset <20 years old)

Manifest HDGEC Participants: Manifest HDGEC Participants must be participants in the Enroll-HD study. Manifest HDGEC Participants will be recruited from English-speaking study sites participating in the Enroll-HD study.

Companion Participants: Companions Participants will be identified for Manifest HDGEC Participants and asked to participate. A Companion Participant's participation for a Manifest HDGEC participant is mandatory.