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. 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 RB1 and WAC (By similarity). May interact with STX1A. Ubiquitously expressed. Expressed in brain, testis, heart, liver and kidney. Weakly expressed in lung, spleen and skeletal muscle (at protein level). Belongs to the BRE1 family. Was originally thought to be cytoplasmic and membrane-associated and to mediate ubiquitination and subsequent degradation of STX1A.