(1) Methods: In a multicenter PPMS cohort (n = 81) with known MRZR status, we measured B cell-activating factor (BAFF), chemokine CXC ligand 13 (CXCL-13), soluble B cell maturation antigen (sBCMA), soluble transmembrane activator and CAML interactor (sTACI), and chitinase-3-like protein 1 (CHI3L1) in the CSF with enzyme-linked immunosorbent assays (ELISAs). The gene discussed is TNFRSF17; the disease is primary progressive multiple sclerosis.