FNDC5 (Fibronectin Type III Domain–Containing Protein 5) is a membrane-associated protein that serves as the precursor of the myokine irisin, a hormone-like peptide involved in energy metabolism and metabolic regulation. FNDC5 belongs to a small group of proteins containing fibronectin type III domains, which are structural motifs commonly involved in cell signaling and extracellular interactions. FNDC5 is expressed primarily in skeletal muscle, but is also found in cardiac muscle, adipose tissue, and other metabolically active tissues. During exercise or metabolic stimulation, FNDC5 can be proteolytically cleaved to release irisin, which circulates in the bloodstream and promotes browning of white adipose tissue, increased thermogenesis, and improved energy expenditure. FNDC5 expression is regulated in part by PGC-1α, a transcriptional coactivator involved in mitochondrial biogenesis and metabolic adaptation to exercise. Because of its role in exercise physiology, metabolic homeostasis, and energy balance, FNDC5/irisin signaling has been widely studied in metabolic disease, obesity, diabetes, and muscle biology, making it an important molecule in endocrine and metabolic research.