FKBP4 and major depressive disorder: It is hypothesized that depressed patients have increased basal levels of FKBP51 (Lukic et al., 2015; Tatro et al., 2009) and has become an important target for physiological stress regulation and potential new drug target therapies for patients with major depressive disorder (Binder et al., 2004; Kirchheiner et al., 2008; Stamm et al., 2016).