The National Institute of Neurological Disorders and Stroke (NINDS) invites applications for ancillary studies that will further our understanding of the unique clinical and biomarker signatures of premanifest or prodromal Huntington's disease (HD) through coordination with the NINDS-funded Neurobiological Predictors of Huntington's Disease (PREDICT-HD) study. The PREDICT-HD study is an international 32-site observational study of persons at-risk for HD, and includes clinical and biospecimen resources from 800 premanifest HD participants and 200 healthy control subjects. Extensive clinical data including UHDRS motor examination, cognitive and neuropsychiatric assessments, and structural MRI imaging have been collected over a ten-year time frame, with a subset of these data accessible through the dbGaP website http://www.ncbi.nlm.nih.gov/projects/gap/cgi-bin/study.cgi?study_id=phs000222.v1.p1. This FOA encourages collaborative investigations combining expertise in imaging, proteomics, biomarker assay development, clinical trial design, phenotype/genotype modifier analysis, and motor, neuropsychological and cognitive assessments of Huntington's disease. Among the areas of research encouraged in this initiative are innovative and informative clinical and biomarker assessments designed to improve the translation of existing knowledge towards efficient and effective clinical trials for the prevention and treatment of Huntington's disease.