FKBP4 and major depressive disorder: Early-life stress, such as physical, sexual, or emotional childhood trauma, has been generally associated with MDD [23, 24], but the increase of MDD risk is also moderated by genetic variants, for example, by FKBP5 variants [25], encoding the FK 506 binding protein 51 or FKBP51, a co-chaperone of the heat-shock protein 90 (hsp90) [26].