FKBP5 and major depressive disorder: As to disentangling the etiologic heterogeneity underlying the clinical diagnoses of depressive and schizophrenic disorders, significant contributions come from (1) recent molecular-genetic studies regarding the FKBP5 gene expression, which appears to predict antidepressant treatment outcome in depression [44, 45], (2) the success of the combined dexamethasone/CRH test as a potential surrogate marker in depression [46, 47],and (3) the description of the potential link between microglial activation and progressive brain changes in schizophrenia [28].