We measured NFL levels in 1138 matching CSF and serum samples collected from 571 participants in 7 diagnostic groups: healthy donors (HD), relapsing-remitting MS (RRMS), primary progressive MS (PPMS), secondary progressive MS (SPMS), clinically isolated syndrome (CIS), noninflammatory neurological diseases (NIND), and other inflammatory neurological diseases (OIND). The gene discussed is NEFL; the disease is secondary progressive multiple sclerosis.