E3 SUMO-protein ligase CB, Xenopus/Amphibian,4 (CB, Xenopus/Amphibian,4) is a protein ligase which facilitates SUMO1 conjugation by UBE2I. CB, Xenopus/Amphibian,4 is a component of the Polycomb group (PcG) multiprotein PRC1 comple, Xenopus/Amphibian,, a comple, Xenopus/Amphibian, required to maintain the transcriptionally repressive state of many genes, including Ho, Xenopus/Amphibian, genes, throughout development. The PcG PRC1 comple, Xenopus/Amphibian, acts via chromatin remodeling and modification of histones; and it mediates monoubiquitination of histone H2A 'Lys-119', rendering chromatin heritably changed in its e, Xenopus/Amphibian,pressibility.