Project Summary/Abstract Most children with neurofibromatosis type 1 (NF1), one of the most common inherited neurological disorders in the United States, experience difficulties with large muscle movements. Current methods of measuring gross motor impairment in young children with NF1 are time- and resource-intensive, reliant on coarse scoring from a proxy typically based on the presence or absence of abilities without regard to the quality of function. These limitations hamper counseling in the clinic on the severity of gross motor difficulties, prediction of future function, and guidance on treatment planning. The objective of this K23 proposal is to develop office-based tools to quantify gait in young children with NF1 that reflect overall gross motor impairment and predict future gross motor difficulties. In this prospective observational study of ambulatory children less than six years old with NF1, we will evaluate gait speed as the fastest time to walk 10 meters and interlimb coordination derived from artificial intelligence-based pose estimates from video. We will conduct office-based gait assessments with video, clinical exams, and neurodevelopmental evaluations during routine medical visits at baseline, 12 months, and 24 months at one of the largest NF1 centers in the U.S., the NYU Comprehensive Neurofibromatosis Center. Using this design, our specific aims are to: (1) evaluate how well gait speed and interlimb coordination reflect gross motor impairment compared to currently available neurodevelopmental assessments; (2) determine the test-retest reliability of gait speed and interlimb coordination; and (3) investigate the prognostic ability of gait speed and interlimb coordination to predict gross motor impairment up to two years later. We expect these findings will result in improved developmental surveillance in clinic and inform clinical trial design with the addition of performance-based endpoints. This proposal aligns with the NINDS strategic plan of