TMEM65 (Transmembrane protein 65) is an inner-mitochondrial-membrane protein essential for calcium homeostasis and organelle integrity. By binding to and stabilizing the Na⁺/Ca²⁺ exchanger NCLX, it facilitates mitochondrial Ca²⁺ efflux, thereby regulating oxidative phosphorylation and limiting ROS production. In cardiomyocytes, TMEM65 localizes to intercalated discs where it interacts with connexin-43, ensuring normal electrical conduction and preventing arrhythmia. Loss-of-function mutations cause mitochondrial myopathy with neurodevelopmental delay, whereas its over-expression in gastric cancer activates the YWHAZ-PI3K-Akt-mTOR axis, promoting tumorigenesis. Thus, TMEM65 links mitochondrial quality control to cytoplasmic signaling, positioning it as a potential therapeutic target in metabolic and oncologic diseases.