Two important signal transduction pathways, the mitogen-activated protein kinases (MAPK)/extracellular signal-regulated kinases (ERKs) and the AKT/mammalian target of rapamycin (mTOR)/P70S6K, are activated by IGF-1 [16,22], which induce MRFs expression and reduce proteolysis and the loss of skeletal muscle mass induced by the ubiquitin-proteasome system [23,24] and the Myostatin (MSTN)/Smad pathway [22], particularly in aging (sarcopenia) [25,26]. The gene discussed is MSTN; the disease is sarcopenia.