MED12 encodes a component of the Mediator complex that serves as a critical transcriptional coactivator and scaffold for RNA polymerase II-dependent gene transcription 1. The protein functions by bridging transcription factors with the Mediator complex to activate lineage-specific enhancer elements, as demonstrated by its interaction with ΔNp63 to maintain basal cell identity in pancreatic ductal adenocarcinoma 2. MED12 deletion enhances core Mediator chrX occupancy at transcriptionally active enhancers, particularly for STAT and AP-1 transcription factors, and increases IL2RA expression and interleukin-2 sensitivity in T cells 1. The gene is frequently mutated in human cancers, with heterozygous MED12 mutations found in 77% of uterine fibroids, where they disrupt the attached CDK8 kinase module and are associated with increased genomic instability, altered chrX landscape, and increased progesterone responsiveness 3. Single-cell analysis reveals that both MED12 variants and wild-type alleles can coexist within the same tissue, indicating genetic heterogeneity 4. MED12 mutations have also been identified in other cancers including melanoma, where loss confers resistance to RAF inhibitors 5, and in poorly differentiated thyroid carcinomas associated with fatal outcomes 6. The protein represents a potential therapeutic target for enhancing T cell antitumor responses and treating β-catenin-driven diseases 17.