Summary of roles of polyP in blood clotting. (A) Long-chain polyP (hundreds to thousands of phosphate units long) acts at 4 points in the clotting cascade, indicated in red: a, initiates the contact pathway of blood clotting^20,21 ; b, accelerates FV activation^20,21 ; c, enhances fibrin polymerization^21,23 ; and d, accelerates FXI activation by thrombin (this study). (B) Platelet-size polyP (60-100mers) acts most potently at 2 points in the clotting cascade, indicated in red: b, accelerates FV activation²¹ ; and d, accelerates FXI activation by thrombin (this study).