APOE and Alzheimer disease: Significantly lower serum BDNF levels in the AD group suggest an insufficient neurotrophic supply. BDNF levels were also significantly lower among all subjects in the AD group with APOE ε4 gene. BDNF possibly interacted with APOE ε4 and co-effects with MMSE scores. BDNF potential contributed to the molecular mechanism of the AD continuum.