In 2011, we showed for the first time that the cNFL concentration was markedly reduced after natalizumab treatment in active MS.24 Thereafter, several studies have shown similar reduction or even normalization of NFL after DMT in both CSF and blood.3,4,9 In our real-world material, we confirm the utility of cNFL as a biomarker for monitoring treatment effect in clinical practice. The gene discussed is NEFL; the disease is myeloid sarcoma.