Deubiquitinase that plays a key regulatory role in the spindle assembly checkpoint or mitotic checkpoint by preventing premature anaphase onset. Acts by specifically mediating deubiquitination of CDC20, a negative regulator of the anaphase promoting complex/cyclosome (APC/C) (PubMed:17443180). Deubiquitination of CDC20 leads to stabilize the MAD2L1-CDC20-APC/C ternary complex (also named mitotic checkpoint complex), thereby preventing premature activation of the APC/C (PubMed:17443180). Promotes association of MAD2L1 with CDC20 and reinforces the spindle assembly checkpoint (PubMed:17443180). Also promotes the deubiquitination of histone H2A and H2B (PubMed:23615962, PubMed:27880911). Recruited to RNF8/RNF168- ubiquitinated chromatin surrounding double stranded breaks (DSBs), promotes hydrolysis of such ubiquitin conjugates, thus negatively regulating protein recruitment to damaged chromatin (PubMed:23615962). Participates in nucleotide excision repair (NER) pathway by deubiquitinating DDB2 to prevent its premature degradation so it can remain on damaged chromatin (By similarity). Promotes FOXP3 stabilization through 'Lys-48'-linked deubiquitination leading to increased stability and increased regulatory T-cell lineage stability (PubMed:32644293). Also plays a positive role in innate immune response to DNA viruses by deubiquitinating STING1, selectively removing its 'Lys-48'-linked polyubiquitin chains and stabilizing it (PubMed:31968013). {ECO:0000250|UniProtKB:Q8C2S0, ECO:0000269|PubMed:17443180, ECO:0000269|PubMed:22681888, ECO:0000269|PubMed:23615962, ECO:0000269|PubMed:27880911, ECO:0000269|PubMed:31968013, ECO:0000269|PubMed:32644293}. This is the function of ENSG00000136014 (USP44, ubiquitin specific peptidase 44).