Additionally, it has recently been shown that in MCI and pre-AD stages, APOE ε4 carriers display higher CSF levels of YKL-40 but not of a marker of microglial activation, sTREM2 (soluble triggering receptor expressed on myeloid cells 2), supporting a role of APOE ε4 in the regulation of astroglial response in AD pathology [93]. Here, APOE is linked to Alzheimer disease.