TMEM127 is a transmembrane protein that functions as a tumor suppressor, primarily through negative regulation of oncogenic signaling pathways. Mechanistically, TMEM127 operates via multiple complementary pathways: it modulates mTOR signaling by cooperating with the early endosomal GTPase Rab5 to inhibit mTOR activity at endolysosomes 1, and it suppresses RET tyrosine kinase signaling by recruiting the NEDD4 E3 ubiquitin ligase to promote RET ubiquitination and lysosomal degradation 2. Additionally, TMEM127 functions in antitumor immunity by forming a complex with SUSD6 and MHC-I that prevents MHC-I surface expression, with therapeutic implications for immune checkpoint blockade resistance 3. Germline TMEM127 mutations account for approximately 2% of hereditary pheochromocytomas and paragangliomas (PCC/PGL), classified within Cluster 2 kinase signaling tumors 4. TMEM127-mutant carriers present predominantly as single benign adrenal tumors in patients over 40 years, with malignancy occurring in ~5% of cases and bilateral disease in 35% 5. Disease-associated mutations disrupt normal intracellular protein distribution and impair RET degradation capacity 5, with clinical penetrance of approximately 13-45% depending on family history status 6. Current clinical guidelines recommend TMEM127 genetic testing in all PCC/PGL cases and annual biochemical surveillance with biennial imaging for carriers 7.