The UHDRS was a clinical rating scale which has been developed by the Huntington Disease Study Group (HSG) to provide a uniform assessment of the clinical features and course of Huntington's Disease (HD). The components of the full UHDRS assess motor function, cognition, behavior and functional abilities. The total motor score (TMS) assessed motor features of HD with standardized ratings of oculomotor function, dysarthria, chorea, dystonia, gait, and postural stability. Some items (such as chorea and dystonia) required grading each extremity (face, bucco-oral-lingual, and trunk) separately. Eye movements require both horizontal and vertical grades. The total motor impairment scores was the sum of all the individual 31 motor sub-items (each rated from 0 to 4), with higher scores indicating more severe motor impairment than lower scores. The range of TMS is 0-124.

An AE was any untoward medical occurrence without regard to causality in a participant who received study drug. An SAE was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly. Treatment-emergent are events between first dose of treatment and up to the follow-up period that were absent before treatment or that worsened relative to pretreatment state. AEs included both SAEs and non-serious AEs.

The following laboratory parameters were analyzed: hematology (hemoglobin, hematocrit, red blood cell \[RBC\] count, platelet count, white blood cell \[WBC\] count, total neutrophils, eosinophils, monocytes, basophils, lymphocytes); blood chemistry (blood urea nitrogen \[BUN\], creatinine, glucose, calcium, sodium, potassium, chloride, total bicarbonate, aspartate aminotransferase \[AST\], alanine aminotransferase \[ALT\], total and direct bilirubin, alkaline phosphatase, uric acid, albumin, and total protein; urinalysis (pH, glucose, protein/albumin, hemoglobin/blood, ketones/acetone, nitrites, leukocyte esterase, microscopy \[if urine dipstick was positive for blood, protein, nitrites or leukocyte esterase\]); others (urine/serum pregnancy test, glycosylated hemoglobin \[HbA1c, if diabetic\]).

Vital signs assessment included pulse rate and blood pressure. Criteria for vital sign values meeting potential clinical concern included: supine/sitting pulse rate less than (\<)40 or greater than (\>)120 beats per minute (bpm), standing pulse rate \<40 or \>140 bpm; systolic blood pressure (SBP) of greater than or equal to (\>=)30 millimeters of mercury (mmHg) change from baseline or SBP \<90 mmHg, diastolic blood pressure (DBP) \>=20 mmHg change from baseline or DBP \<50 mmHg.

ECG parameters included PR interval, QRS complex, and corrected QT interval using Fridericia's formula (QTcF). Criteria for ECG changes meeting potential clinical concern included: PR interval ≥200 milliseconds (msec) or ≥25% increase when baseline is \>100 msec; QRS interval ≥50% increase from baseline when baseline is less than or equal to (\<=)200 msec; and QTcF ≥450 msec or ≥30 msec increase from baseline.

Weight assessment was performed by a study physician or a trained study nurse and was included in the physical examination.

Absolute neutrophil count (ANC) and WBC were monitored for safety. Participants with WBC \<3000 but \>=2000 cells/mm\^3 or ANC \<1500 but \>=1000 cells/mm\^3 were to have study treatment suspended. Participants with WBC \<2000 or ANC \<1000 cells/mm\^3 were to be discontinued from study participation.

The UHDRS is a clinical rating scale to provide a uniform assessment of the clinical features and course of Huntington Disease. The Total Motor Score (TMS) is 1 of the 6 components of UHDRS, includes 31 items, and ranges from a scale of 0 to 124 (higher scores indicate more severe disease).

C-SSRS is a participant rated questionnaire to assess suicidal ideation, suicidal behavior, actual attempts (yes or no responses), and intensity of ideation (rated 1=low severity to 5=high severity). Yes/No responses are mapped to Columbia Classification Algorithm of Suicide Assessment (C-CASA) categories: completed suicide, suicide attempt, preparatory acts towards imminent suicidal behavior, suicidal ideation, and self-injurious behavior, or no suicidal intent. A participant could have a yes or no response in more than 1 category.

C-SSRS is a participant rated questionnaire to assess suicidal ideation, suicidal behavior, actual attempts (yes or no responses), and intensity of ideation (rated 1=low severity to 5=high severity). Yes/No responses are mapped to Columbia Classification Algorithm of Suicide Assessment (C-CASA) categories: completed suicide, suicide attempt, preparatory acts toward imminent suicidal behavior, suicidal ideation, and self-injurious behavior, or no suicidal intent. A participant could have a yes or no response in more than 1 category.

C-SSRS is a participant rated questionnaire to assess suicidal ideation, suicidal behavior, actual attempts (yes or no responses), and intensity of ideation (rated 1=low severity to 5=high severity). Yes/No responses are mapped to Columbia Classification Algorithm of Suicide Assessment (C-CASA) categories: completed suicide, suicide attempt, preparatory acts toward imminent suicidal behavior, suicidal ideation, and self-injurious behavior, or no suicidal intent. A participant could have a yes or no response in more than 1 category.