General initiation factors SL1,TFⅡD and TFⅢB required for initiation of transcription in the three kinds of classic genes in eukaryotic cells respectively are multiprotein complexes that consist of a universal subunit termed TATA-box binding protein (TBP) and a set of TBP associated factors (TAFs).The C terminal domain of TBP, which is highly conserved in a wide variety of species, interacts directly with TATA element or binds to DNA through TAFs. TAFs are polymerase - specific and promoter-specific. In transcriptional initiation，some of the TAFs are expected to function as adoptors for TBP-DNA interaction, some of them may serve as bridging proteins that allow the other TAFs interact with TBP in the assembly of transcription initiation complex，and some as coactivators may mediate transcription initiation activated by sequence-specific activators.