STMP1 is a mitochondrial micropeptide that functions as a short transmembrane protein localized to the inner mitochondrial membrane. Structurally, it shares conserved features with typical respiratory complex subunits, including a small size with one to two transmembrane domains and a charged C-terminal region 1. Primary Function and Mechanism: STMP1 serves as a structural subunit of mitochondrial respiratory chain complexes III and IV, essential for complex assembly and supercomplex formation 2. The protein enhances complex IV (cytochrome c oxidase) activity, which promotes G1/S cell cycle transition by upregulating CCNE2, CDK2, and E2F1 expression 2. Additionally, STMP1 interacts with myosin heavy chain 9 (MYH9) to activate DRP1, promoting mitochondrial fission and redistributing mitochondria to the cell leading edge 3. Disease Relevance: STMP1 is upregulated in multiple malignancies including hepatocellular carcinoma, where elevated levels correlate with poor recurrence-free survival 2. The protein promotes tumor metastasis by enhancing cell migration and lamellipodia formation via DRP1-dependent mitochondrial fission 3. STMP1 expression also associates with Paget's disease of bone through genetic regulatory mechanisms in osteoclasts 4, and altered expression appears in the brains of multiple sclerosis patients with sex-specific differences 5. Clinical Significance: STMP1 represents a potential therapeutic target for cancer metastasis prevention and cell cycle-related diseases, with DRP1 inhibitors showing promise in preclinical models 3.