Transcription initiation factor IIA (TFIIA) is a heterotrimer, the three subunits being known as alpha, beta, and gamma, in order of molecular weight. The N and C-terminal domains of the gamma subunit are represented in pfam02268 and pfam02751, respectively. This family represents the precursor that yields both the alpha and beta subunits. The TFIIA heterotrimer is an essential general transcription initiation factor for the expression of genes transcribed by RNA polymerase II. Together with TFIID, TFIIA binds to the promoter region; this is the first step in the formation of a pre-initiation complex (PIC). Binding of the rest of the transcription machinery follows this step. After initiation, the PIC does not completely dissociate from the promoter. Some components, including TFIIA, remain attached and re-initiate a subsequent round of transcription.