CHI3L1 and Brain atrophy: Based on the above-mentioned findings of a strong association between high CSF CHI3L1 levels and the development of neurological disability and brain atrophy in CIS patients, the question that then arises is whether CHI3L1 is just a mere biomarker that can be measured in the CSF and reflects the degree of astrocyte activation, or CHI3L1 could be neurotoxic per se.