MED15 is a core component of the Mediator complex, a multi-subunit transcriptional coactivator essential for RNA polymerase II-dependent gene expression 1. As a Mediator subunit, MED15 facilitates preinitiation complex (PIC) assembly and enhances RNA polymerase II elongation by bridging gene-specific regulatory proteins to the basal transcription machinery 2. MED15 specifically interacts with tissue-specific transcription factors; in pancreatic β-cells, it binds Nkx6-1 and NeuroD1 to regulate maturation genes 3, while in cancer contexts it associates with oncogenic transcription factors like YAP/TAZ and TFE3 2, 4. MED15 regulates multiple signaling pathways including TGF-β and HIF signaling, with the latter creating a positive feedback loop during hypoxia 5, 6. Clinically, MED15 overexpression associates with aggressive cancer phenotypes: it correlates with poor prognosis and recurrence in head and neck squamous cell carcinoma 5, and MED15-TFE3 fusions drive renal cell carcinoma with uncertain prognosis 7. MED15 also interacts extensively with huntingtin, modulating Mediator assembly with potential implications for neurodegeneration 8. These findings position MED15 as both a critical transcriptional regulator and promising biomarker/therapeutic target across multiple disease contexts.