The somatomedin B domain of vitronectin (VTN) binds to and stabilizes plasminogen activator inhibitor-1 (PAI1) (Declerck et al. 1988). PAI1 is the principal physiological inhibitor of both tissue (tPA) and urokinase (uPA) plasminogen activators and a key regulator of the fibrinolytic system; the stabilization of PAI1 by VTN thereby regulates proteolysis of fibrin (Zhou et al. 2003). Elevated PAI1 activity is associated with coronary thrombosis (Hamsten et al. 1987) and poor prognosis in many cancers.