TM9SF3 (transmembrane 9 superfamily member 3) is a Golgi-resident transmembrane protein that functions as a selective autophagy receptor essential for Golgiphagy—the selective degradation of Golgi fragments 1. TM9SF3 binds all six mammalian ATG8 proteins through multiple N-terminal LC3-interacting regions (LIRs), thereby directing fragmented Golgi membranes into autophagosomes for lysosomal degradation 2. This process is critical under nutrient stress and various Golgi-stress conditions, including exposure to monensin, brefeldin A, and glycosylation perturbations 1. Mechanistically, TM9SF3 maintains Golgi integrity and glycosylation fidelity by promoting degradation of aberrantly glycosylated proteins; loss of TM9SF3 or mutations in its LIRs compromise protein glycosylation 2. TM9SF3 regulates high mannose N-glycan levels by controlling Golgi morphology and function 3. Clinically, TM9SF3 has significant disease relevance in cancer. High TM9SF3 expression is required for breast cancer cell proliferation and correlates with poor prognosis in both luminal and triple-negative breast cancers 14. TM9SF3 mediates FUCA2-induced triple-negative breast cancer aggressiveness, with elevated expression associated with advanced TNM stage, lymph node metastasis, and shortened overall survival 4. TM9SF3 is also upregulated in lung adenocarcinoma and regulated by miR-195-3p, suggesting roles in multiple cancer types 56.