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

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…