Biomarker Expression in Patients With ACTH-Dependent Cushing's Syndrome Before and After Surgery

Inclusion Criteria: * ≥ 18 years of age. * Has a documented diagnosis of ACTH-dependent, endogenous Cushing's syndrome and is scheduled for curative surgery. * Must be able to comprehend and sign an approved Informed Consent Form (ICF) and other applicable study enrollment documents. Exclusion Criteria: * Plans for pre-operative and/or intra-operative use of glucocorticoid ("steroid cover"). * Use any of the following treatments for Cushing's syndrome, as specified: * 4 weeks prior to first specimen collection and/or during the study period. * Adrenostatic medications (metyrapone, ketoconazole, fluconazole, aminoglutethimide, LCI699 or etomidate etc). * Short-acting somatostatin analogs (octreotide, pasireotide). * 6 weeks prior to first specimen collection and/or during the study period. o Mifepristone. * 8 weeks prior to first specimen collection and/or during the study period. o Neuromodulator drugs that act at the hypothalamic-pituitary level: serotonin antagonists (cyproheptadine, ketanserin, retanserin), dopamine agonists (bromocriptine, cabergoline), gamma-aminobutyric acid agonists (sodium valproate), and somatostatin receptor ligands (octreotide long-acting release \[LAR\], pasireotide LAR, lanreotide). * Concomitant use of the following due to their potential to stimulate the expression of FKBP5: * Testosterone or other steroid hormone analogues. * Oral contraceptives or hormonal replacement therapy. * History of illness that the Principal Investigator (PI) considers could interfere with or affect the conduct, results, and/or completion of the clinical trial. * Pregnancy or breastfeeding.