RYBP (RING1- and YY1-binding protein) is a member of the polycomb group (PcG). This protein interacts with MDM2 and decreases MDM2-mediated p53 ubiquitination, leading to stabilization of p53 and an increase in p53 activity. RYBP induces cell-cycle arrest and is involved in the p53 response to DNA damage. RYBP has been shown to interact with RING1, YY1, Caspase 10, E2F3, E2F2, Mdm2, Abl gene and CBX2.