A ubiquitin ligase complex consisting of Bre1 and Rad6 that mediates monoubiquitination of histone H2B to form H2BK123ub1. H2BK123ub1 gives a specific tag for epigenetic transcriptional activation, elongation by RNA polymerase II, telomeric silencing, and is also a prerequisite for H3K4me and H3K79me formation. It thereby plays a central role in histone code and gene regulation. It also modulates the formation of double-strand breaks during meiosis. [ GOC:bhm http://www.ncbi.nlm.nih.gov/pubmed/19531475 ]