May function as a ubiquitin-protein or polyubiquitin hydrolase involved both in the processing of ubiquitin precursors and of ubiquitinated proteins. May therefore play an important regulatory role at the level of protein turnover by preventing degradation of proteins through the removal of conjugated ubiquitin. Essential component of TGF-beta/BMP signaling cascade. Deubiquitinates monoubiquitinated SMAD4, opposing the activity of E3 ubiquitin-protein ligase TRIM33. Monoubiquitination of SMAD4 hampers its ability to form a stable complex with activated SMAD2/3 resulting in inhibition of TGF-beta/BMP signaling cascade. Deubiquitination of SMAD4 by USP9X re-empowers its competence to mediate TGF-beta signaling (By similarity). Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Interacts with SMAD4. Additional isoforms seem to exist. Widely expressed in embryonic and adult tissues. USP9Y is located in the 'azoospermia factor a' (AZFa) region on chromosome Y which is deleted in Sertoli cell-only syndrome. This is an infertility disorder in which no germ cells are visible in seminiferous tubules leading to azoospermia. However, AZFa deletions resulting in complete loss of USP9Y have also been found in normospermic men (PubMed:19246359). The disease may be caused by variants affecting the gene represented in this entry. The role of USP9Y in spermatogenesis failure is uncertain (PubMed:19246359). A 4-bp deletion in a splice-donor site, causing exon skipping and protein truncation has been observed in non-obstructive azoospermia (PubMed:10581029). However, complete USP9Y deletion has been detected in individuals with no spermatogenic defects (PubMed:19246359). May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the peptidase C19 family.