The function of USP39 (ubiquitin specific peptidase 39, Ensembl gene identifier ENSG00000168883) is as follows. Deubiquitinating enzyme that plays a role in many cellular processes including cellular antiviral response, epithelial morphogenesis, DNA repair or B-cell development (PubMed:33127822, PubMed:34614178). Plays a role in pre-mRNA splicing as a component of the U4/U6-U5 tri-snRNP, one of the building blocks of the precatalytic spliceosome (PubMed:11350945, PubMed:26912367). Specifically regulates immunoglobulin gene rearrangement in a spliceosome-dependent manner, which involves modulating chromatin interactions at the Igh locus and therefore plays an essential role in B-cell development (By similarity). Regulates AURKB mRNA levels, and thereby plays a role in cytokinesis and in the spindle checkpoint (PubMed:18728397). Regulates apoptosis and G2/M cell cycle checkpoint in response to DNA damage by deubiquitinating and stabilizing CHK2 (PubMed:30771428). Also plays an important role in DNA repair by controlling the recruitment of XRCC4/LIG4 to DNA double-strand breaks for non-homologous end-joining repair (PubMed:34614178). Participates in antiviral activity by affecting the type I IFN signaling by stabilizing STAT1 and decreasing its 'Lys-6'-linked ubiquitination (PubMed:33127822). Contributes to non-canonical Wnt signaling during epidermal differentiation (By similarity). Acts as a negative regulator NF-kappa-B activation through deubiquitination of 'Lys-48'-linked ubiquitination of NFKBIA (PubMed:36651806). {ECO:0000250|UniProtKB:Q3TIX9, ECO:0000269|PubMed:11350945, ECO:0000269|PubMed:18728397, ECO:0000269|PubMed:26912367, ECO:0000269|PubMed:30771428, ECO:0000269|PubMed:33127822, ECO:0000269|PubMed:34614178, ECO:0000269|PubMed:36651806}.