Deubiquitinating enzyme that regulates the degradation of various proteins by removing ubiquitin moieties, thereby preventing their proteasomal degradation. Stabilizes RNF123, which promotes CDKN1B degradation and contributes to cell proliferation (By similarity). Decreases the levels of ubiquitinated proteins during skeletal muscle formation and acts to repress myogenesis. Modulates transcription of major myofibrillar proteins. Also involved in turnover of endoplasmic- reticulum-associated degradation (ERAD) substrates (PubMed:19465887, PubMed:24356957). Mechanistically, deubiquitinates and thereby stabilizes several E3 ligases involved in the ERAD pathway including SYVN1 or MARCHF6 (PubMed:24356957). Regulates the stability of other E3 ligases including BIRC2/c-IAP1 and BIRC3/c-IAP2 by preventing their ubiquitination (PubMed:21849505). Required for cells to mount an appropriate response to hypoxia by rescuing HIF1A from degradation in a non-catalytic manner and by mediating the deubiquitination of FUNDC1 (PubMed:22128162, PubMed:33978709). Attenuates mitochondrial damage and ferroptosis by targeting and stabilizing NADPH oxidase 4/NOX4 (PubMed:38943386). Negatively regulates TNF- and IL-1beta-triggered NF- kappa-B activation by hydrolyzing 'Lys-27'- and 'Lys-63'-linked polyubiquitin chains from MAP3K7 (PubMed:31127032). Modulates also the protein level and aggregation of polyQ-expanded huntingtin/HTT through HSP90AA1 (PubMed:33094816). {ECO:0000250|UniProtKB:Q3UJD6, ECO:0000250|UniProtKB:Q6J1Y9, ECO:0000269|PubMed:19465887, ECO:0000269|PubMed:21849505, ECO:0000269|PubMed:22128162, ECO:0000269|PubMed:22689415, ECO:0000269|PubMed:24356957, ECO:0000269|PubMed:31127032, ECO:0000269|PubMed:33094816, ECO:0000269|PubMed:33978709, ECO:0000269|PubMed:38943386}. This is the function of USP19 (ubiquitin specific peptidase 19, Ensembl gene identifier ENSG00000172046).