BDNF and Alzheimer disease: Approximately 30% of the population carry the BDNF Met66 allele (Shen, et al., 2018), which is associated with reduced activity-dependent secretion of BDNF and specific impairments in hippocampal-dependent encoding and retrieval processes, hippocampal volume, and episodic memory in both cognitively normal samples (Egan, et al., 2003; Erickson, et al., 2010; Hariri, et al., 2003) and across the spectrum of AD.