Early study of serum versus CSF MOG-IgG testing reported CSF positivity in only two-thirds of seropositive patients, suggesting higher sensitivity of serum testing.22 Yet subsequent work found that, while overall sensitivity of serum MOG-IgG testing was higher than CSF, isolated CSF MOG-IgG positivity occurred in 3/44 (7%) with non-MS inflammatory demyelinating disease.23 This raised the question of whether a subset of patients with MOGAD may harbor antibodies that are only detectable in CSF and motivated investigation into the utility of CSF MOG-IgG testing. Here, MOG is linked to myeloid sarcoma.