Recently, cerebrospinal fluid (CSF) YKL-40 levels were reported to be a 
promising candidate biomarker of glial inflammation in Alzheimer's disease (AD). 
To detect how APOE ε4 affects CSF YKL-40 levels in cognitively normal (CN) 
states, mild cognitive impairment (MCI) and AD dementia, data from 35 CN 
subjects, 63 patients with MCI, and 11 patients with AD from a cross-sectional 
study in the Alzheimer's Disease Neuroimaging Initiative (ADNI) database were 
investigated. The results showed that CSF YKL-40 concentrations were increased 
in the AD dementia group than in the CN group. CSF YKL-40 levels were higher in 
APOE ε4 carriers than in noncarriers with MCI. No statistically significant 
difference was found in CSF YKL-40 levels between APOE ε4 carrier and 
noncarriers in AD and CN subjects. CSF YKL-40 concentrations were tightly 
related to CSF tau and p-tau concentrations in the MCI group. Analysis implied 
that APOE ε4 might affect CSF YKL-40 levels in MCI subjects, suggesting a 
crucial role of APOE ε4 in neuroinflammation in detecting individuals who might 
convert to AD from MCI and, thus, as an effective predictive factor.