TRIM9 is an E3 ubiquitin-protein ligase that functions as a critical regulator of neuronal development and a potential tumor suppressor across multiple cancer types. Structurally, TRIM9 catalyzes K11-linked ubiquitination and proteasomal degradation of target proteins through its RING domain 1. In neurons, TRIM9 plays a key role in axon branching by regulating SNARE-mediated exocytosis; it directly interacts with the Netrin-1 receptor DCC and negatively regulates SNAP25 availability for SNARE complex formation in the absence of Netrin-1 stimulation 2. Beyond neurodevelopment, TRIM9 demonstrates tumor-suppressive functions in multiple cancer contexts. In pancreatic cancer, TRIM9 downregulation correlates with poor prognosis, and its overexpression suppresses cell proliferation and migration through HNRNPU degradation 1. Similarly, in esophageal cancer, TRIM9 is downregulated and functions as a tumor suppressor by promoting ZEB1 protein degradation via the ubiquitin-proteasome pathway 3. TRIM9 also functions as an oncogene in non-small cell lung cancer, where it is upregulated and promotes EMT; miR-218-5p negatively regulates this axis 4. In drug-resistant hepatocellular carcinoma, TRIM9 inhibition enhances doxorubicin sensitivity 5. Clinically, TRIM9 autoantibodies represent high-risk paraneoplastic biomarkers associated with cerebellar syndrome and metastatic cancer 67.