Optimised assays, and now the ability to measure NfL levels in blood, have increased its potential for translation to clinical practice.13–18 However, MS is a heterogeneous disease, with subtypes and trajectories, in flux between relapse, remission, stability and progression.19 Age, gender and comorbidities can influence any potential biomarker, and DMTs (with differing mechanisms of action) are widely prescribed. Here, NEFL is linked to myeloid sarcoma.