TMEM209 is a transmembrane nuclear envelope protein that functions as an integral component of the nuclear pore complex (NPC). It serves as a novel transmembrane nucleoporin that interacts with NUP205 and Nup210 to facilitate nuclear transport 1. Through its association with NUP205, TMEM209 stabilizes this nucleoporin and increases nuclear c-Myc levels, promoting nuclear transport of MYC and other proteins 2. TMEM209 is normally restricted to testis but becomes aberrantly expressed across multiple cancer types. In lung cancer, TMEM209 overexpression drives cell proliferation and survival; ectopic overexpression promotes growth while attenuation blocks it 2. The protein also localizes to the Golgi apparatus and cytoplasm in cancer cells 2. TMEM209 depletion impairs cell cycle progression by delaying entry into S, G2, and M phases 1. Clinically, TMEM209 serves as a prognostic biomarker in esophageal squamous cell carcinoma, where it predicts immunotherapy response and overall survival 3. Additionally, NUP205 knockdown reduces TMEM209 expression and enhances chemosensitivity in hepatocellular carcinoma 4, suggesting the TMEM209-NUP205 axis as a potential therapeutic target across multiple malignancies.