FKBP5 and major depressive disorder: To determine the age at which FKBP51 levels were sufficient to cause depression-like behavior and whether this phenotype was dependent on FKBP51 across lifespan, separate cohorts of wild-type and FKBP5−/− mice were examined at 6, 10, and 21 months of age for depression-like behavior in the forced swim test (FST) and tail suspension test (TST) and stress reactivity via post-restraint CORT sampling.