CDK8 is a serine/threonine kinase that functions as a component of the Mediator complex, a large coactivator required for regulated transcription of RNA polymerase II-dependent genes 1. As part of the dissociable CDK8 kinase module (CKM), CDK8 represses Mediator-dependent transcription by positioning its catalytic domain to obstruct RNA polymerase II recruitment through an intrinsically disordered region in MED13 2. CDK8 phosphorylates the C-terminal domain of RNA polymerase II and CCNH, modulating transcription initiation and complex formation 3. Additionally, CDK8 phosphorylates STAT1 to regulate inflammatory signaling and facilitates NOTCH degradation through interaction with MAML1. Clinically, CDK8 dysregulation associates with multiple malignancies. Elevated CDK8 expression correlates with shorter relapse-free survival across breast cancer subtypes and predicts therapy failure 4. MED12 mutations disrupting the CDK8 kinase module occur in 77% of uterine fibroids and promote tumor growth 5. Conversely, CDK8/19 inhibition enhances antitumor T cell responses by increasing IL-2 sensitivity and effector function 6, and facilitates conversion of conventional T cells to stable regulatory T cells for immunosuppressive therapy 7. These findings identify CDK8 as both an oncogenic driver and a therapeutic target.