However, the association of BDNF levels in the brain or peripheral blood with cognitive impairment in AD is quite complex and influenced by many factors, such as sample heterogeneity and methodological differences (Kim et al., 2017; Qin et al., 2017; Balietti et al., 2018; Ng et al., 2019; Marta et al., 2020; Xie et al., 2020; Girotra et al., 2022). The gene discussed is BDNF; the disease is Alzheimer disease.