FNDC5 (fibronectin type III domain containing 5) is a membrane-bound protein that serves as the precursor for irisin, an exercise-induced myokine 1. Upon physical activity, PGC-1α activates FNDC5 expression in skeletal muscle, leading to ectodomain cleavage and irisin release 2. Irisin exerts pleiotropic effects by binding to αV/β5 integrin receptors 3, inducing white adipose tissue browning through UCP1 expression and improving metabolic homeostasis 1. Beyond metabolism, FNDC5/irisin mediates exercise's neuroprotective effects through a PGC-1α-dependent pathway that increases hippocampal BDNF expression, rescuing synaptic plasticity and memory deficits in Alzheimer's disease models 42. The gene also regulates bone remodeling through effects on osteocytic survival and sclerostin production 3. Circulating irisin levels are markedly reduced in obesity, muscle atrophy, osteoporosis, cardiovascular disease, and Alzheimer's disease, but elevated in cancer 1. Recent evidence demonstrates that recombinant irisin administration ameliorates age-associated sarcopenia and metabolic dysfunction in mice, improving grip strength, insulin sensitivity, and reducing hepatic steatosis 5. These findings establish FNDC5/irisin as a critical molecular mediator of exercise's systemic health benefits, positioning it as a potential therapeutic target for metabolic and neurodegenerative diseases.