TAF3 is a core promoter-binding subunit of the TFIID basal transcription factor complex essential for RNA polymerase II transcription initiation 1. As part of the TFIID-A module alongside TAF5 and TBP, TAF3 recognizes H3K4me3-marked promoters through its plant homeodomain (PHD) finger, facilitating preinitiation complex assembly 2. TAF3's H3K4me3-binding activity is further enhanced by serotonylation of histone H3Q5, a modification that stabilizes H3K4me3 and potentiates its readout 34. Beyond canonical transcription initiation, TAF3 exhibits specialized developmental functions. During myogenic differentiation, TAF3 forms a complex with TBPL2 that selectively replaces TFIID at specific promoters 5. In zebrafish development, selective TAF3-TRF3 interaction is required for hematopoiesis and early embryogenesis, distinct from TBP-dependent functions 6. In embryonic stem cells, TAF3 regulates lineage commitment through long-range chr10 looping with CTCF/cohesin complexes 7. Clinically, TAF3 represents a negative regulator of p53 transcription function 8, with elevated expression inhibiting p53-dependent gene activation. TAF3 is implicated in hepatocellular carcinoma pathogenesis, where elevated expression correlates with poor prognosis and cell proliferation 9. These findings establish TAF3 as a multifunctional transcriptional regulator integrating epigenetic signals with developmental and stress-response programs.