Component of the RNF20/40 E3 ubiquitin-protein ligase complex that mediates monoubiquitination of Lys-120 of histone H2B (H2BK120ub1). H2BK120ub1 gives a specific tag for epigenetic transcriptional activation and is also prerequisite for histone H3 Lys-4 and Lys-79 methylation (H3K4me and H3K79me, respectively).