BDNF and Alzheimer disease: The interaction between BDNF Met and APOE4 has a weak effect on amyloid-β plaque burden, and the longitudinal PET measurements of Alzheimer’s disease-related carriage have a weak effect on the decline in the cerebral metabolic rate for glucose (CMRgl) in cognitively unimpaired late-middle-aged and older adults, but there is no apparent effect on the rate of cognitive decline.