Inclusion Criteria for HD Participants:  Estimated at low or high probability of motor diagnosis based the multivariate risk score (MRS) Willing to commit to two in-person assessment visits (baseline and 2 year follow-up) and one remote assessment (1 year follow-up) No active comorbidities (i.e. receiving stable treatment) All medications will be allowed although the protocol will mandate documentation of medications and analyses will particularly assess potential impact of medications on outcomes (i.e., sedation of abnormal movements) CAG results must be 36 and above as measured in genetic tests already completed  Inclusion Criteria for Healthy Controls (HC):  Willing to commit to two in-person assessment visits (baseline and 2 year follow-up) and one remote assessment (1 year follow-up) In generally good health IQ > 70 Able to undergo an MRI scan  Exclusion Criteria (for all Participants):  Evidence of unstable medical or psychiatric illness (including substance abuse) History of severe learning disability, mental retardation, or other central nervous system (CNS) disease or event (e.g., seizures, head trauma, additional neurological diagnoses) Treatment with phenothiazine-derivative antiemetic medications such as prochlorperazine, metoclopramide, promethazine and Inapsine greater than 3 times per month History of serious alcohol or drug abuse within the past year Unable (determined by patient's prescribing doctor) to not take tryptophan, leucine, niacin or niacinamide-containing dietary supplements, anti-inflammatory medications, anti-coagulants (such as warfarin and heparin) or anti-platelets (such as aspirin) in the past 14 days to assure safety during lumbar puncture Unable to fast (no food or drink, only water) overnight before the lumbar puncture