Furthermore, brain-derived neurotrophic factor (BDNF) and transforming growth factor-beta1 (TGF-β1) are implicated in the pathophysiology of depression and cognitive impairment, with the BDNF Val66Met allele linked to late-life depression and the BDNF 11757C allele associated with depression in AD, and the C/C phenotype of TGF-β1 linked to an increased risk of AD and depression in AD patients [48,49,50,51,52,53]. Here, BDNF is linked to major depressive disorder.