TBP is a subunit of the eukaryotic general transcription factor TFIID. TFIID is the first protein to bind to DNA during the formation of the transcription preinitiation complex of RNA polymerase II (RNA Pol II). As one of the few proteins in the preinitiation complex that binds DNA in a sequence-specific manner, it helps position RNA polymerase II over the transcription start site of the gene. However, it is estimated that only 10–20% of human promoters have TATA boxes. Therefore, TBP is probably not the only protein involved in positioning RNA polymerase II. Majority of human promoters are TATA-less housekeeping gene promoters. The binding of TBP to these promoters is facilitated by housekeeping gene regulators. Interestingly, transcription initiates within a narrow region at around 30 bp downstream of TATA box on TATA-containing promoters, while transcription start sites of TATA-less promoters are dispersed within a 200 bp region.