The function of RNF40 (ring finger protein 40, ENSG00000103549) is as follows. Component of the RNF20/40 E3 ubiquitin-protein ligase complex that mediates monoubiquitination of 'Lys-120' of histone H2B (H2BK120ub1). H2BK120ub1 gives a specific tag for epigenetic transcriptional activation and is also prerequisite for histone H3 'Lys-4' and 'Lys-79' methylation (H3K4me and H3K79me, respectively). It thereby plays a central role in histone code and gene regulation. The RNF20/40 complex forms a H2B ubiquitin ligase complex in cooperation with the E2 enzyme UBE2A or UBE2B; reports about the cooperation with UBE2E1/UBCH are contradictory. Required for transcriptional activation of Hox genes. {ECO:0000269|PubMed:16307923, ECO:0000269|PubMed:19410543}. (Microbial infection) Promotes the human herpesvirus 8 (KSHV) lytic cycle by inducing the expression of lytic viral genes including the latency switch gene RTA/ORF50. .