MUSTN1 (musculoskeletal embryonic nuclear protein 1) is a small nuclear microprotein (~9.6 kDa) that functions as a key developmental regulator of musculoskeletal tissues 1. In skeletal muscle, MUSTN1 is expressed by Pax7-positive satellite cells and is essential for myoblast proliferation, differentiation, and fusion 23. The protein is transcriptionally regulated by AP family transcription factors (c-Fos, Fra-2, JunD) and MyoD1 14. Beyond nuclear functions, MUSTN1 is secreted from smooth muscle cells as an exosome-transmissible microprotein that modulates skeletal muscle extracellular matrix composition 54. Mechanistically, MUSTN1 prevents muscle atrophy by suppressing ferroptosis through ACO1-dependent iron homeostasis and lipid peroxidation reduction 4. MUSTN1 also interacts with SMPX to regulate myogenic differentiation and stabilizes FABP3 to promote adipogenesis 36. Clinically, MUSTN1 expression is dynamically elevated in response to exercise, muscle injury, and regeneration 72. MUSTN1 knockout mice display reduced muscle mass, impaired regeneration, altered fiber-type composition, decreased glucose metabolism, and reduced exercise endurance 28, while protection against diet-induced obesity 6. These findings establish MUSTN1 as a critical regulator with therapeutic potential for muscle-wasting diseases and metabolic disorders.