Fluid biomarkers associated with disease activity and progression, particularly neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP) in CSF, have been demonstrated to reflect distinct pathological processes, with NfL capturing acute neuroaxonal injury and GFAP reflecting astrocytic activation, capturing some features of chronic smouldering compartmentalized inflammation.12,13 However, the relationship between these markers and specific immune cell populations in different MS phenotypes and disease states remains poorly understood. The gene discussed is GFAP; the disease is myeloid sarcoma.