Amino acids of PF4 important for heparin binding are largely conserved among species. Alignment of the heparin binding region (R20, R22, H23, K46, R49, K61, K62, K65, and K66) of PF4, IL-8, and an uncharacterized protein (n/a) and total amino acid similarity of various species in percentages compared with human (100%) is shown. Above the amino acid sequence of 1 PF4 monomer (70 amino acids, primary structure without signal sequence, UniProt P02776), the crystal structure of a PF4 tetramer (quaternary structure, PDB code 1f9q) is depicted from the front (left) and from the top view (right). Lysine residues of the C-terminus (K61, K62, K65, and K66) are highlighted in green; arginine (R20, R22, and R49), histidine (H23), and other lysine residues (K46) are highlighted in blue. The pictures of the crystal structure of PF4 were made using PyMOL Molecular Graphics System Version 1.3 Schrödinger LLC.