There are many causes of cerebellar ataxia (CA) including neurodegeneration and autoimmunity. We detected antiseizure-related 6 homolog-like 2 (Sez6l2) antibody from the serum of a patient with subacute CA.1 In this study, we conducted an 11-year follow-up of a previously reported female (case 1)1 and reported two new cases (case 2 and case 3) of Sez6l2 antibody-positive CA in Japan (online supplemental table 1). Sez6l2 antibody was identified by cell-based assay and immunoblot analysis using FLAG-tagged Sez6l2 protein in 2 (case 2 and case 3) of the 162 patients with CA but in none of the 1 healthy control or 78 cases of degenerative diseases (figure 1A, online supplemental figure 1, online supplemental material and methods). The sera of the two patients with Sez6l2 antibody showed the same patterns of reactivity in tissue-based immunofluorescence assay (TBA) (figure 1B). Analysis of serum IgG subclasses showed that all of the sera with Sez6l2 antibody had both IgG1 and IgG4 immunoreactivity