Study to Measure Cerebrospinal Fluid Mutant Huntingtin Protein in Participants With Early Manifest Stage I or Stage II Huntington's Disease

Inclusion Criteria: * Capacity to consent to participate in the study as assessed using the Evaluation to Sign Consent tool and investigator judgment * Age 25 to 65 years, inclusive, at the time of signing Informed Consent Form * Early manifest, Stage I or Stage II HD (defined as TFC of 7-13, inclusive) * Genetically confirmed disease (CAG repeat length ≥ 36 in huntingtin gene by direct DNA testing) * Body mass index ≥18 and ≤32 kg/m2; total body weight \>50 kg * Ability to undergo and tolerate MRI scans * Ability to tolerate blood draws and lumbar puncture * Ability and willingness to comply with all aspects of the protocol, including completion of interviews and questionnaires and carrying/wearing of a digital monitoring device * Stable medical, psychiatric, and neurological status for at least 12 weeks prior to screening and at the time of enrollment * Signed study companion consent for participation, if a study companion is available * For women of childbearing potential: agreement to remain abstinent or use acceptable contraceptive methods during the observational period Exclusion Criteria: * Any condition, including severe chorea, that would prevent either writing or performing pen and paper or smartphone-based tasks * History of attempted suicide or suicidal ideation with plan (i.e., active suicidal ideation) that required hospital visit and/or change in level of care within 12 months prior to screening * Current active psychosis, confusional state, or violent behav…