Platelet factor 4, also known as CXCL4, is expressed in megakaryocytes and stored in the a-granules of platelets. Recombinant human PF-4 is a 7.8 kDa protein containing 70 amino acid residues, including the four highly conserved residues present in CXC chemokines. Platelet factor 4 can be antiproliferative and antiangiogenic, at least in part via interfering with FGF2 and VEGF heparin binding and thus inhibiting their signaling. However, it can also be proinflammatory and proatherogenic through multiple effects on monocytes, macrophages and endothelial cells.