A Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of RG6496 in Huntington's Disease

This is a first-in-human (FIH) study of RG6496 that will assess the safety and tolerability of single-ascending doses of RG6496 administered to huntington's disease gene expansion carriers (HDGECs). The study consists of two parts: Part 1 \[single-ascending dose\] followed by Part 2 \[open-label extension (OLE)\].

Inclusion Criteria: Part 1 * Confirmation of HDGEC status with cytosine-adenine-guanine (CAG) expansion \> 39. * Confirmation of SNP carrier status of the target SNP * Independence Scale (IS) score of ≥70, total functional capacity (TFC) ≥10, total motor score (TMS) \>6. * Ability to read the words "red," "blue," and "green" and be fluent in the language of the informed consent form (ICF) and the tests used at the study site. * Ability to walk unassisted. * Total body weight \> 40 kilogram (kg) and body mass index (BMI) within the range 18-32 kilogram per square meter (kg/m\^2) (inclusive) at baseline. * Ability to undergo and tolerate MRI scans. Part 2 * Completed the post-dose safety follow-up period in the Part 1 of the study. * In the opinion of the Investigator, the participant has not experienced a worsening in health that precludes their safe continued participation in the study. Exclusion Criteria: Part 1 * Concurrent or planned participation in any interventional clinical study, including current use of an ASO or any HTT-lowering therapy or treatment with investigational therapy within 90 days or 5 drug-elimination half-lives, whichever is longer prior to screening * Pregnant or breastfeeding, or with the intention of becoming pregnant during the study or within the timeframe in which contraception is required * Malignancy within 5 years prior to screening * Planned brain surgery during the study * Positive HIV test, hepatitis B surface antigen and hepatitis B surface antigen at screening * Active psychosis, confusional state, or violent behavior, including aggression that could cause harm to self or others, over the 12 weeks prior to screening. * Current or previous history of a primary independent psychotic disorder. * Scoliosis or spinal deformity or surgery making IT injection not feasible in an outpatient setting * 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. Part 2 * Prematurely discontinued from Part 1 for any reason (i.e., before the completion of the postdose safety follow-up period of Part 1). * Pregnant or breastfeeding, or with the intention of becoming pregnant during the study or within the timeframe in which contraception is required. * Concurrent or planned participation in any interventional clinical study, including current use of an ASO or any HTT-lowering therapy * Received any active investigational treatment other than RG6496 during or since completion of Part 1 of the study. * Had confirmed Dose-Limiting Adverse Event (DLAE)(s) in Part 1 of the study.