The purpose of this study is to compare: 1) spatial and temporal gait measures, 2) performance on functional mobility measures, and 3) amount of daily walking activity before and after administration of Xenazine in individuals with HD. It is hypothesized that the use of Xenazine to decrease chorea will improve functions of 1) gait, 2) sit-to-stand transfers 3) stair climbing and 4) overall daily physical activity and function.