In a recently published meta-analysis of observational studies, serum BDNF levels in patients with Alzheimer’s disease (AD) and individuals with mild cognitive impairment (MCI) in comparison to healthy controls have been examined and the results showed a reduced serum BDNF level in patients with AD compared to healthy subjects; but no significant relationship between MCI and serum BDNF levels was found40. Here, BDNF is linked to early-onset autosomal dominant Alzheimer disease.