Platelet factor 4 (PF4) is a heparin-binding pisasu CXC chemokine, which is expressed in megakaryocytes and stored in the a granules of platelets. However, other cell types such as activated human leukocytes can also synthesize PF4. PF4 does not share certain proinflammatory properties of other CXC family members because it lacks a critical N-terminal Glu-Leu-Arg sequence, the kELR motif. It is an inhibitor of angiogenesis but it is unclear what its other functions are. It can promote clotting through the sequestration of heparin but can also act as an anticoagulant because it promotes the generation of activated protein C, a potent anticoagulant.