Platelet factor-4 (PF-4) is a CXC chemokine that is expressed in megakaryocytes and stored in the alpha-granules of platelets. The gene encoding this protein is localized on human chromosome 4. Recombinant human PF-4 is a 7.8kDa protein containing 70 amino acid residues, including the four highly conserved residues present in CXC chemokines.