Molecular model of the PF4 tetramer derived from x-ray crystallography. The C-terminal lysines 61, 62, 65, 66 are shown in dark blue. Other lysines, arginines, and histidines contributing to the circumferential belt of positive charges around the PF4 tetramer are shown in cyan. Relevant to this report, the proline 37 amino acids are shown in red.