RNA polymerase II general transcription factor TFIID is a macromolecular complex comprising the TATA-binding protein, TBP and 13-14 evolutionary conserved TBP-associated factors, TAFs. Although genetic experiments have shown that TAFs are essential for cell cycle progression in yeast and in rapidly proliferating vertebrate cells in vitro, new experiments indicate they may be dispensible in specific developmental and physiological processes. Moreover, the TAF4 subunit of TFIID negatively regulates proliferation by inhibiting activation of the TGFbeta signalling pathway by its paralogue TAF4b. TAF4 is however essential in the retinoic acid and cAMP signalling pathways acting as a cofactor for CREB and the retinoic acid receptor, but is a negative regulator of the ATF7 transcription factor.