Rb may act as a transcriptional co-activator in undifferentiated F9 cells.

The reversible interaction of the retinoblastoma susceptibillty gene product (Rb) with the cellular transcription factor E2F has recently been demonstrated. Activation of the adenovirus E2a promoter by the products of the viral E1a gene correlates with the ability of both early E1a proteins to sequester Rb, thereby releasing E2F from inactive complexes with this protein. The E2a promoter is also efficiently stimulated by a product (17.5 kDa) of the viral E4 gene. The specific interaction of this E4 protein with E2F results in the formation of complexes that bind cooperatively to the two neighboring E2F binding sites in the E2a promoter