Component of a Polycomb group (PcG) multiprotein PRC1-like complex, a complex class required to maintain the transcriptionally repressive state of many genes, including Hox genes, throughout development. PcG PRC1 complex acts via chromatin remodeling and modification of histones; it mediates monoubiquitination of histone H2A 'Lys-119', rendering chromatin heritably changed in its expressibility. Enables methylated histone binding activity. Involved in negative regulation of transcription by RNA polymerase II. Located in chromatin and nucleoplasm. Part of PRC1 complex. Biomarker of esophagus squamous cell carcinoma and glioblastoma.