GFAP and primary progressive multiple sclerosis: As a prospective study that is well-powered for pwPPMS, CYPRO is a novel approach accounting for the distinct features of pwPPMS, that besides PPMS-relevant performance indices and patient-reported outcomes measures, includes blood-derived biomarkers, such as sNfL or GFAP, that are closely related to neurodegenerative aspects of PPMS pathophysiology.