Any alteration in this peculiar expression pattern results in myelination defects; marked GPR17 upregulation and/or accumulation of GPR17-expressing cells at the border of demyelinated lesions has been observed in a wide variety of pathological conditions associated with dysmyelination, including patients affected by MS [97,98], traumatic brain injury [99] and congenital leukoencephalopathy [100]. Here, GPR17 is linked to myeloid sarcoma.