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). It thereby plays a central role in histone code and gene regulation. The RNF20/40 complex forms a H2B ubiquitin ligase complex in cooperation with the E2 enzyme UBE2A or UBE2B; reports about the cooperation with UBE2E1/UBCH are contradictory. Required for transcriptional activation of Hox genes. Recruited to the MDM2 promoter, probably by being recruited by p53/TP53, and thereby acts as a transcriptional coactivator. Mediates the polyubiquitination of PA2G4 leading to its proteasome-mediated degradation. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Component of the RNF20/40 complex (also known as BRE1 complex) probably composed of 2 copies of RNF20/BRE1A and 2 copies of RNF40/BRE1B. Interacts with UBE2E1/UBCH6. Interacts with p53/TP53 and WAC. Interacts with PAF1; the interaction mediates the association of the PAF1 and RNF20/40 complexes which is a prerequsite for recruitment of UBE2A/B. Interacts with PA2G4. Belongs to the BRE1 family.