RNF20 is an E3 ubiquitin ligase protein that mediates monoubiquitination of histone H2B and the methylation of histone H3. It forms a ubiquitin ligase complex in cooperation with the E2 enzyme UBE2E1/UBCH6. It thereby plays a central role in histone code and gene regulation. It is required for transcriptional activation of Hox genes. It is recruited to the MDM2 promoter, probably by being recruited by p53/TP53, and thereby acts as a transcriptional coactivator.