Initiation of transcription from protein-coding genes in eukaryotes is a complex process that requires RNA polymerase II, as well as families of basal transcription factors. Binding of the factor TFIID (TBP) to the TATA box is believed to be the first step in formation of the preinitiation complex (PIC) which contains several additional factors, including TFIIA, TFIIB, TFIIE, TFIIF and TFIIH. Recognition of the TATA binding element by TBP may be regulated by TFIIA. TFIIA consists of three subunits designated TFIIA-伪, TFIIA-尾 and TFIIA-纬, all of which associate with both TBP and TAF (TBP-associated factor). TFIIA functions to stabilize the interaction between TFIID and DNA by binding directly to TBP and the DNA (at the TATA box), thus forming a TBP/TFIIA/TATA complex which mediates the transcriptional output of a gene.