To this point, biomarkers of neuroaxonal and glial damage/dysfunction such as tau or GFAP, have been linked to progression more specifically than NfL (70–72) and would provide clarity on whether late disease or perhaps “later life” progression is classified currently as SPMS and PPMS largely share pathological processes or, alternatively, might represent completely distinct manifestations. Here, MAPT is linked to primary progressive multiple sclerosis.