Interactions between heparan sulfate proteoglycans and the partner proteins regulate many aspects of cell functions, however, their binding properties are still equivocal. Developed eight years ago was a method for selective labelling of lysine residues in the heparin binding sites of Fibroblast Growth Factors (FGFs). The labelling of arginine residues is far more challenging, due to the multiple reaction products between a dicarbonyl and a guanidino group.