A tetrapeptide found on fibronectin, fibrinogen α, and von Willebrand factor, but not vitronectin or collagen; interacts with α5β1 and αVβ3 integrins, blocking the attachment of cells to fibronectin-coated surfaces (Ki = 0.6 mM); inhibits adhesion of platelets to fibronectin, fibrinogen α, and von Willebrand factor (IC50 = ~10 μM)