TAF9 is a core component of the TFIID basal transcription factor complex essential for RNA polymerase II-dependent transcription initiation 1. As part of TFIID, TAF9 facilitates promoter recognition and pre-initiation complex assembly 1. Beyond TFIID, TAF9 participates in multiple transcriptional regulatory complexes including TFTC, PCAF, and STAGA, where it functions in both transcriptional activation and repression of distinct gene sets 2. TAF9 is essential for cell viability 2 and plays roles in cell differentiation processes including nervous system development and myogenesis 3. Mechanistically, TAF9 acetylation status regulates TFIID recruitment; HDAC1-mediated TAF9 deacetylation promotes TFIID assembly and gene activation, while acetylated TAF9 prevents TFIID binding 4. TAF9 functions as a shared coactivator competed for by transcription factors like p53 and GLI1, with TAF9 binding critical for GLI oncogenic activity 5. Clinically, TAF9 dysregulation associates with disease. Molecular mimicry between cytomegalovirus pp65 and TAF9 triggers cross-reactive autoantibodies in systemic lupus erythematosus patients, correlating with glomerulonephritis and proteinuria 67. TAF9 overexpression in hepatoblastoma promotes epithelial-mesenchymal transition and poor prognosis through the lncRNA938-TAF9-TTK axis, representing a potential therapeutic target 8.