A 68-year-old Korean man (proband; subject II.1 in Fig. 1b), who exhibited slowly progressive gait disturbance with tendency to fall for several months, was referred to our hospital. Albeit self-ambulatory, the proband suffered from unsteady gait due to ataxia. His medical history was unremarkable except for mild hypertension. The proband had no family history of neurological diseases or consanguineous marriage. Neurological examination revealed dysphagia, dysarthria, dysphonia, wide-based truncal ataxia, bilateral gaze-evoked nystagmus and exaggerated deep tendon reflexes with bilaterally positive Babinski and Hoffman signs, indicating dysfunctions in the brainstem, cerebellum or cervical cord. The proband did not present with sensory disturbance, palatal myoclonus, and abnormal mental or emotional status. To determine the etiology of ataxia, extensive workup was carried out including somatosensory evoked potential recording, blood and urine chemistry tests, assay of serum levels of vitamin B12 and thyroid hormones, venereal disease research laboratory (VDRL) test, anti-human immunodeficiency virus (HIV) antibody test and genetic studies for spinocerebellar ataxia (types 1, 2, 3, 6, 7 and 8), Friedreich ataxia and dentato-rubro-pallido-luysian atrophy. All of these tests were negative, however.