SYNJ2 encodes an inositol 5-phosphatase that regulates phosphoinositide metabolism and membrane trafficking, with roles in clathrin-mediated endocytosis and synaptic vesicle dynamics. The enzyme dephosphorylates phosphatidylinositol lipids including PIP₂ and PIP₃, and may mediate inhibitory effects of Rac1 on endocytosis. SYNJ2 is upregulated in multiple cancers including lung squamous cell carcinoma, papillary thyroid carcinoma, hepatocellular carcinoma, and gastric cancer, where elevated expression correlates with poor prognosis 123. In gastric cancer, SYNJ2 promotes metastasis and angiogenesis through interaction with GRB2 4. Conversely, SYNJ2 dysfunction is clinically relevant in hearing disorders: rare variants in SYNJ2 associate with age-related hearing loss 5, and mouse mutations cause progressive high-frequency hearing loss through mechanisms affecting outer hair cell maintenance 6. Recent evidence suggests SYNJ2 regulates mitochondrial quality control by anchoring Pink1 mRNA to mitochondria in a metabolic-dependent manner, placing PINK1 activation under insulin signaling control 7. Polymorphisms in SYNJ2 also associate with cognitive abilities in aging populations 8. The dual role of SYNJ2 in cancer progression and sensory/neurological maintenance reflects its broader importance in phosphoinositide-dependent cellular processes.