Methods and materials herein are useful for assessing and/or treating autoimmune ataxias associated with septin-specific autoantibodies (e.g., septin-5-specific autoantibodies). For example, septin-5 polypeptides are provided as well as methods for using septin-5 polypeptides to detect septin-5-specific autoantibodies and/or to treat a mammal having an autoimmune ataxia.