In the late stage of MCI or early stage of dementia, the continuous decrease in BDNF may stimulate the compensatory mechanism (increased platelet-derived BDNF synthesis and release) (Fujimura et al., 2002), resulting in increased plasma BDNF levels, and in the late stage of dementia, compensatory failure will reduce blood BDNF levels. The gene discussed is BDNF; the disease is dementia.