text: There are at least three distinct mechanisms in which pRb can repress transcription of E2F-regulated promoters. Though these mechanisms are known, it is unclear which are the most important for the control of the cell cycle.
  E2Fs are a family of proteins whose binding sites are often found in the promoter regions of genes for cell proliferation or progression of the cell cycle. E2F1 to E2F5 are known to associate with proteins in the pRb-family of proteins while E2F6 and E2F7 are independent of pRb. Broadly, the E2Fs are split into activator E2Fs and repressor E2Fs though their role is more flexible than that on occasion. The activator E2Fs are E2F1, E2F2 and E2F3 while the  repressor E2Fs are E2F4, E2F5 and E2F6. Activator E2Fs along with E2F4 bind exclusively to pRb. pRb is able to bind to the activation domain of the activator E2Fs which blocks their activity, repressing transcription of the genes controlled by that E2F-promoter.