APP and major depressive disorder: On a genetic level, our included studies have identified certain genes related to anxiety (e.g., BDNF, GABRA1, GAD1, SLC6A4, 5-HT3A, AKT1, FOS, and APP); these are in concordance with a recent systematic review suggesting that BDNF, PERIOD2, and SLC6A4 were correlated, not only with anxiety, but with depression and addictions as well [60].