Fig. 8. Model for TGF-β-dependent transcriptional activation of the p15^Ink4B gene. An oligomeric, likely trimeric, complex consisting of Smad2, Smad3 and Smad4 is translocated into the nucleus upon TGF-β stimulation. Smad3 contacts DNA at the SBE on the p15^Ink4B promoter, to which Sp1 was already bound at the Sp1 site (only one Sp1 binding site and one SBE are shown). Additional protein–protein interactions occur between Sp1 and Smad2, Sp1 and Smad4, and Smad2 and/or 3 and CBP/p300. TAF/TBP represents TATA-binding protein (TBP) and its associated factors (TAF), the transcription factors for general transcription associated with RNA polymerase II.