In contrast, studies of PPMS display a wide spectrum of CSF-NFL concentrations, including cases with low (< 500 pg/ml) (4, 5) or not detectable CSF-NFL concentrations (6), and cases with high CSF-NFL levels (>10,000 pg/ml) (7) comparable with the documented ranges in relapsing remitting MS disease courses (7–9), painting a heterogeneous picture (10, 11). Here, NEFL is linked to primary progressive multiple sclerosis.