RBBP5 (UniProtKB/Swiss-Prot entry Q15291) belongs to the highly conserved family of WD-repeat proteins. It’s part of the MLL1/MLL complex and is as such involved in mono-, di- and trimethylation at lysine 4 of histone H3, a specific tag for epigenetic transcriptional activation. RBBP5 binds to the retinoblastoma protein thereby playing a role in the regulation of cell proliferation. It also plays a crucial role in the differentiation potential in embryonic stem (ES) cells.