Huntington's Disease Young Adult Study 2.0

The goal of this observational study is to learn about the first signs of disease in young adult carriers of the gene for Huntington's disease. The main questions to answer are: * what are the earliest signs of the disease? * can we identify the best time to intervene with treatment to prevent or delay onset of symptoms? * can we identify the most reliable markers of disease for use in prevention trials? Participants will undergo the following assessments: * clinical examination * cognitive and neuropsychiatric testing * brain imaging * biofluid sampling Researchers will compare gene carriers with matched controls to see if any of these measures show evidence of early disease effects.