NEFL and myeloid sarcoma: In relapsing-remitting MS, NfL is associated with clinical and magnetic resonance imaging (MRI) inflammatory activity and predicts disability accrual.10, , , , –15 CSF NfL levels increase during relapses, correlating with disability and MRI lesion load.16,17 However, in progressive MS, disease activity is modest, while neurodegeneration is relentless.