At clinical examination, the patient's mental status was normal. The patient was unable to walk or sit alone. Head titubation and truncal sway were present while sitting. Static and kinetic cerebellar syndromes were patent, with bilateral dysmetria in both arms and legs. Oculomotor examination showed intense vertical nystagmus and diminution in the speed of horizontal and vertical saccades. No motor or sensory deficit was present. There was no clinical evidence of an underlying tumor. Magnetic resonance imaging (MRI) of the brain showed diffuse abnormal hyperintensity in the whole cerebellum on fluid-attenuated inversion recovery and diffusion sequences (Figure, A), with no abnormalities on T1-weighted (Figure, B), T1-weighted gadolinium-enhanced, and T2-weighted images. Magnetic resonance angiography was normal. Results of the biological workup were negative for general inflammation, vitamin deficiency, and bacterial or viral infections. Analysis of the cerebrospinal fluid (CSF) showed pleiocytosis (190/mm3 with a lymphocyte predominance), elevated protein level (72 mg/dL [to convert to grams per liter, multiply by 0.01]), glucose level and IgG index (0.6) within the reference ranges, and no oligoclonal IgG bands. Based on the MRI and CSF analysis results, a wide-spectrum anti-infectious treatment for possible infectious cerebellitis was started. However, CSF cultures were negative for bacterial or fungal infections, and polymerase chain reaction found no evidence of herpesvirus family infection. Consequently, anti-infectious drug therapy was stopped after 10 days. No clinical improvement was observed during this period. For detecting onconeural antibodies, indirect immunochemistry was performed on rat brain sections. It demonstrated staining for Purkinje cell bodies and the molecular layer of the cerebellum using the serum and CSF of the patient (screening dilution for serum, 1:20 000; for CSF, 1:500) (Figure, E). The pattern of labeling was different from that seen with anti-Tr antibodies. In addition, the serum of the patient did not inhibit the immunostaining of a biotinylated anti-Tr IgG.4 Results of Western blotting using onconeural recombinant proteins (Hu, Yo, Ri, CV2, amphiphysin, Ma2, and Homer 35) were negative. Finally, mGluR1-Abs were detected in the serum and CSF by means of abolishment of the typical staining pattern on sections of mGluR1-knockout mice, as previously described (not shown).2 To confirm the mGluR1 specificity of these antibodies, a cell-based assay was developed. Briefly, the coding sequence of mouse mGluR1 was cloned in a green fluorescent protein expression vector (GeneCopoeia, Rockville, Maryland). For the cell-based assay, human embryonic kidney cells (HEK293) were grown on glass coverslips in Dulbecco modified Eagle medium (Gibco, Grand Island, New York) with 10% fetal calf serum. After 24 hours, cells were transfected using a transfection reagent (Lipofectamine LTX; Invitrogen, Cergy-Pontoise, France) with the mGluR1 green fluorescent protein–tagged expression construct. Twenty-four hours after transfection, the cells were fixed with 4% paraformaldehyde for 10 minutes and then incubated in a saturation buffer (phosphate-buffered saline, 0.2% gelatin and 0.01% Triton). Cells were then incubated with the patient's CSF (1:50) for 90 minutes. Cells were subsequently washed in phosphate-buffered saline and then incubated with cy3-conjugated antihuman IgG. The CSF of the patient was positive for mGluR1 antibody, whereas CSF samples from 10 patients with other cerebellar degeneration (2 with anti–glutamic acid decarboxylase antibodies, 2 with anti-Tr antibodies, 1 with anti-Yo antibodies, and 5 without known antibodies) were negative for mGluR1 antibody on the mGluR1 cell-based assay (Figure 1F-H). Results of whole-body computed tomography, fludeoxyglucose F 18 (18F)–positron emission tomography, and mammograms were normal. Serum β2-microglobulin and lactic dehydrogenase assays and bone marrow aspiration yielded negative findings.