FKBP5 and post-traumatic stress disorder: Thus, we tested whether: 1) social defeat increases CeA Fkbp5 expression, 2) Fkbp5 expression predicts CeA activation responses to defeat, as defined by the immediate-early genes (IEG) Fos, FosB, ΔFosb, Egr1, Egr2, Jun, or JunB. Social defeat was studied as a stressor because, in humans, social defeat, including being bullied, mobbed, or losing social rank, predicts PTSD, anxiety, and depression symptoms (Bjorkqvist, 2001; Siddaway et al., 2015; Tatar and Yuksel, 2019; Troop and Hiskey, 2013).