NEFL and multiple sclerosis: A Letter of Support was granted to the International Progressive multiple sclerosis Alliance (https://www.progressivemsalliance.org/) by the US Food and Drug Administration (FDA; https://www.fda.gov/media/149608/download) in June 2021 to encourage further study and use of NfL in plasma or serum as a pharmacodynamic/response biomarker for early clinical trials in progressive multiple sclerosis.