Studies in relapsing MS have shown that CSF NfL levels are increased during relapses and are positively associated with disability and MRI lesion load, particularly gadolinium enhancement.15,25 However, in a cohort like the one we recruited in the MS-SMART trial, where relapses occurred in a small proportion of patients (about 10%) before and during the study, and with a median number of T2 new/enlarged lesions of 0 (interquartile range (IQR) = 0–6) over 96 weeks,20,22 the likelihood of observing a change in NfL in relation to disease activity is expected to be low. The gene discussed is NEFL; the disease is myeloid sarcoma.