PSMD9 (proteasome 26S subunit, non-ATPase 9) is a multifunctional chaperone protein with primary roles in proteasome assembly and beyond. As a canonical function, PSMD9 acts as an assembly chaperone during 26S proteasome base subcomplex formation, participating in the intermediate PSMD9:PSMC6:PSMC3 modulator trimer complex before being released during final assembly 1. Recently identified functions extend far beyond proteasome biology. PSMD9 was discovered as a lipid regulatory protein involved in controlling mammalian lipid metabolism and hepatosteatosis pathogenesis 2. In cancer contexts, PSMD9 expression is elevated and prognostically significant across multiple malignancies. In glioblastoma, PSMD9 knockdown inhibits cell proliferation, invasion, migration, and induces G2/M arrest, with high expression conferring resistance to panobinostat 3. In hepatocellular carcinoma, PSMD9 promotes progression by interacting with E3 ubiquitin ligase c-Cbl to suppress EGFR ubiquitination and activate ERK1/2/Akt signaling, with PSMD9 knockdown sensitizing cells to erlotinib 4. In metastatic prostate cancer, PSMD9 participates in an m6A-modified enhancer RNA regulatory complex affecting radiotherapy resistance 5. PSMD9 also shows associations with immune responses and proteostasis regulation, with elevated serum levels in long-term responders to immunotherapy 6 and identified as a risk factor for osteosarcopenia 7. The protein's multifunctionality relies primarily on protein-protein interactions rather than enzymatic activity 8.