Polycomb group (PcG) proteins are chromatin-associated proteins which are important for embryonic and adult stem cell self-renewal and maintenance. At least two distinct human PcG complexes have been identified. Polycomb repressive complex I (PRC1) contains CBX4/Pc2, HPH1 and RING domain-containing proteins (RING1, RING2 and BMI1). CBX4 (Chromobox protein homolog 4 or PC2), which functions as a transcriptional suppressor, is a candidate of KyoT2-binding proteins. It is a E3 SUMO-protein ligase which facilitates SUMO1 conjμgation. Depletion of CBX4 results in decreased cellular resistance to ionizing radiation. CBX4 is directly involved in the cellular response to DNA damage.