NEFL and relapsing-remitting multiple sclerosis: In the case of individuals with established MS, some authors have reported elevated NfL as the sole indicator of disease activity in people with progressive MS (PMS) vs relapsing remitting MS (RRMS) (Reyes et al., 2020), while others have found greater levels of NfL in RRMS vs PMS (Martin et al., 2019), and that NfL levels at time of diagnosis correlated with long- term progression from RRMS to PMS (Bhan et al., 2018).