(formerly termed somatomedin C), the most important somatomedin for postnatal growth; it is produced in the liver, kidney, muscle, pituitary, the gastrointestinal tract, and chondrocytes. IGF-I is a basic protein (7600 MW) that circulates bound to six distinct IGF binding proteins (IGF-BPs), which increase the half-life of circulating IGF-I to 3-18 hours, as compared with the half-life of 20-30 minutes for unbound hormone. Local tissue generation of IGF-I/SM-C, particularly in bone, may play an important role in growth mediation through its paracrine effects. SYN: somatomedins.