The transcription, a complete set of general transcription factors and RNA polymerase need to be assembled at the core promoter to form the ~2.5 million Dalton preinitiation complex.  For example, for promoters that contain a TATA box near the TSS, the recognition of TATA box by the TBP subunit of TFIID initiates the assembly of a transcription complex. The next proteins to enter are TFIIA and TFIIB, which stabilize the DNA-TFIID complex and recruit Pol II in association with TFIIF and additional transcription factors. TFIIB serves as the bridge between the TATA-bound TBP and the RNA polymerase. It also helps to place the active centre of the polymerase in the correct position to initiate transcription. One of the last transcription factors to be recruited to the preinitiation complex is TFIIH, which plays an important role in promoter melting and escape.