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). This evidence concerns the gene NEFL and primary progressive multiple sclerosis.