Major transport protein for glucocorticoids and progestins in the blood of almost all vertebrate species. Produced and secreted by hepatocytes, but has also been identified in a number of glycocorticoid responsive cells (it is found in maternal lung, spleen, and ovary and fetal kidney). Proteolytic cleavage leads to an important conformation change. This reduces the affinity for steroids (By similarity). Belongs to the serpin family.