LETMD1 (LETM1 domain containing 1) is a mitochondrial outer membrane protein that plays crucial roles in metabolic regulation and cellular homeostasis. In brown and beige adipose tissue, LETMD1 is essential for thermogenesis, functioning through both UCP1-dependent and UCP1-independent energy expenditure pathways 12. The protein enhances mitochondrial biogenesis by stimulating PPARGC1A transcription and regulates creatine futile cycle genes 2. LETMD1 maintains mitochondrial structure and calcium homeostasis, with its deletion causing mitochondrial dysfunction, BAT whitening, and metabolic disorders including obesity and glucose intolerance 1. Beyond metabolic functions, LETMD1 negatively regulates immune responses in macrophages by suppressing LPS-induced inflammation, phagocytosis, and NF-κB activation through modulation of mitochondrial ROS production 3. In cardiovascular health, LETMD1 protects against atherosclerosis by reducing endothelial inflammation and pyroptosis 4. The protein also participates in mitochondrial K+/H+ exchange, working with other mitochondrial carriers to maintain proper ion homeostasis 5. LETMD1's diverse functions make it a potential therapeutic target for metabolic diseases, with compounds like SP-8356 showing promise in enhancing its beneficial effects 2.