text: Transcription factor II H (TFH) is an important protein complex, having roles in transcription of various protein-coding genes and DNA nucleotide excision repair (NER) pathways. TFH first came to light in 1989 when general transcription factor-δ or basic transcription factor 2 was characterized as an indispensable transcription factor in vitro. This factor was also isolated from yeast and finally named TFH in 1992.
  TFH consists of ten subunits, 7 of which (ERCC2/XPD, ERCC3/XPB, GTF2H1/p62, GTF2H4/p52, GTF2H2/p44, GTF2H3/p34 and GTF2H5/TTDA) form the core complex. The cyclin-activating kinase-subcomplex (CDK7, MAT1, and cyclin H) is linked to the core via the XPD protein. Two of the subunits, ERCC2/XPD and ERCC3/XPB, have helicase and ATPase activities and help create the transcription bubble.  In a test tube, these subunits are only required for transcription if the DNA template is not already denatured or if it is supercoiled.
  Two other TFH subunits, CDK7 and cyclin H, phosphorylate serine amino acids on the RNA polymerase II C-terminal domain and possibly other proteins involved in the cell cycle. Next to a vital function in transcription initiation, TFH is also involved in nucleotide excision repair.