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Pituitary adenomas constitute up to 20% of primary brain tumors, yet somatic mutations are only found in 15% of pituitary adenomas. Epigenomic dysregulation has been proposed as a tumorigenic mechanism in pituitary adenomas causing Cushing's disease (CD). We created paired datasets of human CD adenomas and en-route margin adult human pituitary glands, and assayed their chromatin accessibility, DNA methylation, transcriptomic, proteomic, and phospho-proteomic landscapes. In CD adenomas, we found epigenetic reactivation of a neurodevelopmental phosphoprotein program typically lost in the postnatal pituitary gland. CD cells overexpressed PPP1R17, an endogenous inhibitor of the protein phosphatase PP2A. Mechanistically, PPP1R17 overexpression in normal murine pituitary cells recapitulated the adenoma phenotype, and the resulting hyperproliferation was reversed in a PP2A-dependent manner by fingolimod, an FDA-approved small molecule, both in vitro and in vivo. Our findings highlight kinase-phosphatase imbalance as a targetable mechanism in CD.