The most promising soluble biomarker in MS is neurofilament light (NfL),15 a marker of axonal damage that can be determined in cerebrospinal fluid (CSF)16 as well as in blood.17,18 There is accumulating evidence that NfL is a reliable biomarker of disease activity in RRMS,19,20 that may also reflect therapeutic efficacy.21 Consequently, NfL has served as an additional outcome measure in clinical trials.22 This evidence concerns the gene NEFL and myeloid sarcoma.