**5.7 YY1**

The transcriptional repressor YY1 can bind to target sites adjacent to E2F binding elements in the promoters of genes such as *Cdc6* (Schlisio et al., 2002). In addition, the YY1 accessory protein Ring1 and YY1 binding protein (RYBP) can interact with E2F-2, -3 and -4 to synergistically enhance binding of E2F-2 and -3 (but not of E2F-1). In this manner, YY1 and RYBP not only enhance the binding and transcription of E2F to certain promoters, but also add specificity.
