Transcription Initiation Factor IIB (TFIIB) is an essential factor for transcription by RNA Polymerase II. TFIIB localizes to the nucleus where it forms a complex (the DAB complex) with transcription factor IID and IIA. TFIIB plays a role as a bridge between IID, which initially recognizes the promoter sequence, and RNA polymerase II. TFIIB is involved in the selection of transcription start site.