FKBP4 and depressive symptom measurement: The FK506 binding protein 51 (FKBP51) is important for the regulation of the stress response, and polymorphisms of the FKBP51-encoding gene, FKBP5, have been associated with atypical stress axis reactivity, with high levels of FKBP51 protein associated with a hyperreactive stress pathway often seen in patients with depression.5,43 Supporting these observations, deletion of FKBP5 in mice leads to a reduction in corticosterone secretion and anxiety-related behaviours.17,18 FKBP51 is a co-chaperone that changes folding and activity of other proteins.