Involved in transcriptional activation. Also required for ubiquitination of histone H2B to form H2BK123ub1. H2BK123ub1 gives a specific tag for epigenetic transcriptional activation, telomeric silencing, and is also a prerequisite for H3K4me and H3K79me formation. Its precise role in H2BK123ub1 formation however is unclear (By similarity).