MED21 is a component of the Mediator complex, a conserved coactivator essential for RNA polymerase II-dependent transcription 1. The MED21-MED7 heterodimer forms a critical hinge region in the Mediator middle module that is required for binding to RNA polymerase II and holoenzyme assembly; point mutations in this hinge increase disorder and markedly reduce Pol II affinity 1. MED21 participates in multiple cellular processes: it promotes keratinocyte proliferation and differentiation through vitamin D receptor signaling 2, regulates plant defense against necrotrophic fungi through histone modifications 3, and is necessary for HIV-1 transcription 4. In pathological contexts, MED21 exhibits elevated expression in hepatocellular carcinoma correlating with poor prognosis and is associated with immune cell infiltration patterns 5. Conversely, MED21 expression is downregulated in coronary heart disease and chr12 obstructive pulmonary disease, where it functions as a biomarker for disease diagnosis 67. These findings establish MED21 as a versatile transcriptional coregulator with roles spanning developmental processes, immune responses, viral replication, and disease pathogenesis.