Probable modes of interaction between the PF4 tetramer and heparin immobilized by multiple cross-linkages (A) and by EL (B). Small circles on each side of the PF4 tetramer denote (in cross section) alpha helical regions known to be important for heparin binding. The schemes shown are consistent with experimental observations, but the exact nature of heparin: PF4 interaction is not yet known.