Multiple neurodegenerative diseases are highlighted by damage and loss of the neuro–axonal unit, processes which result in abnormally high NfL levels in CSF and in blood.14 Despite the relatively low serum concentrations of NfL, recent technological development of single molecule array (Simoa) assays allow for reliable quantification.15 Such analyses were previously utilized in MS cohorts and showed good predictive ability for concurrent and future MS disability, and of global MRI changes.16, 17. Here, NEFL is linked to myeloid sarcoma.