∙ PwHDs and HC participants:

• Female and male adults, aged 21-75 years old, inclusive.

• Adequate visual (Snellen chart) and auditory (Rinne and Weber tests) acuity to complete the psychological testing as determined by the investigator.

• Capable of giving informed consent.

• Willing to comply with highly effective contraceptive measures following informed consent (for Cohort 2 only).

• Vital signs within certain set ranges.

• Considered by the investigator to be in good health as judged by the absence of clinically significant diseases, laboratory values, physical examination, and able to travel to imaging and clinical assessment centers in London, UK.

• Suitable physically and psychologically to travel (with a companion if requested) and undergo the assessments as judged by the investigator.

∙ PwHDs without symptoms: (approximately HD-ISS stage 0 or 1)

• HDGECs with ≥ 40 CAG repeats

• TMS ≤ 6 AND TFC ≥ 12 AND CAP \> 70 PwHDs with symptoms in early disease: (approximately HD-ISS stage 2)

• HDGECs with ≥ 40 CAG repeats

• If one of the following criteria is met:

‣ TMS ≤ 6 AND TFC = 11

⁃ TMS is between 7 and 23 inclusive AND TFC is between 11 and 13 inclusive

⁃ TMS is between 24 and 33 inclusive AND SDMT \> 50 AND TFC is between 11 and 13 inclusive PwHDs with symptoms in late disease (approximately HD-ISS stage 3)

• HDGECs with ≥ 40 CAG repeats

• If one of the following criteria is met:

‣ TMS ≤ 6 AND TFC is between 7 and 10 inclusive

⁃ TMS is between 7 and 23 inclusive AND TFC is between 8 and 10 inclusive

⁃ TMS is between 24 and 33 inclusive AND SDMT \> 50 AND TFC is between 7 and 10 inclusive

⁃ TMS is between 7 and 23 inclusive AND TFC = 7

⁃ TMS \> 23 AND SDMT ≤ 50 AND TFC is between 7 and 13 inclusive

∙ Healthy Controls (HC):

• Age- and sex-matched, and balanced (±8 years) with PwHDs.

• No known family history of HD or have known family history of HD but have been tested for the huntingtin gene glutamine codon (CAG) expansion and are not at genetic risk for HD (CAG \< 36).