A 58-year-old man presented with 1 month subacute onset of instability, oscillopsia, vertigo, nausea, and vomiting. Neurologic examination revealed gaze-evoked and prominently downbeat nystagmus, left eyelid ptosis, and gait cerebellar ataxia (Videos 1 and Videos 2). There was no dysarthria or dysmetria. Head MRI was unremarkable and lumbar puncture showed mild lymphocyte predominant pleocytosis with normal proteins. EMG showed no abnormalities. Cancer was not identified. Serum and CSF septin-5–specific immunoglobulin G was identified by rat cerebellar immunohistochemistry and confirmed with in-house cell-based assay (HEK293 cells transfected with human Sep5) and Western blot (Figure).1,2 Immunotherapy using corticosteroids, IV immunoglobulin, cyclophosphamide, and rituximab showed minimal improvement.