RAB39A is a small GTPase that functions as a key regulator of intracellular membrane trafficking by cycling between GDP-bound inactive and GTP-bound active states to recruit downstream effectors 1. Its primary role involves regulating autophagosome-lysosome fusion through recruitment of the HOPS endosomal tethering complex onto lysosomes, where lysosomal RAB39A and autophagosomal RAB2A coordinate HOPS subcomplexes to mediate membrane tethering and SNARE-driven fusion 2. RAB39A localizes to the late endocytic pathway at multivesicular bodies and controls trafficking of sphingomyelin, phospholipids, and lysobisphosphatidic acid 3. It negatively regulates LPS-induced autophagosome formation in macrophages through PI3K interaction 4, and facilitates caspase-1-dependent IL-1β secretion by serving as a trafficking adaptor 5. RAB39A plays roles in phagosome maturation and pathogen handling. Clinically, RAB39A exhibits context-dependent effects in cancer: it promotes cancer stemness and tumorigenesis through the RAB39A-RXRB axis in sarcomas and lymphoid malignancies 6, yet acts as an epigenetically silenced tumor suppressor in cervical cancer by inhibiting AKT-mediated EMT 7. Dysregulation implicates RAB39A in neurodevelopmental disorders, as defective DENND5B variants affecting RAB39A interaction cause cognitive impairment and white matter abnormalities 8.