Histone H2B lysine 120 monoubiquitination (H2BK120ub) is a key histone modification that plays critical roles in gene transcriptional regulation and higher order chromatin organization in many species1. H2BK120ub has been reported to be associated with highly expressed active genes in human cells2. In yeast, H2B lysine 123 monoubiquitination (the homolog of mammalian H2BK120ub) is a determinant of two important histone H3 methylations, H3K4 methylation and H3K79 methylation1,3. Histone H3K4me3 is a well-defined gene activation marker. Both H3K4 and H3K79 methylation significantly affect chromatin structure and gene transcription. In a recent study, the presence of H2B monoubiquitination was also found to stimulate H3K4me3 during gene transcription in mammals4, which further supports a conserved role for H2B monoubiquitination in transcription.

RAD6 is a well-characterized E2 ubiquitin-conjugating enzyme for H2B ubiquitination, and BRE1 (RNF20) is its corresponding E3 ligase4,5. In our recent studies, we found that Rad6 also targets the degradation of p53 in both Drosophila and mammals6,7. The molecular regulation of RAD6/BRE1 and H2BK120ub has been thoroughly investigated5,8. However, the biological functions of H2BK120ub and its related enzymes in stem cell differentiation are poorly understood.

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