After their synthesis and secretion, BMP2 and 4 interact with proteoglycans, which are present both at the cell surface and within the extracellular matrix. In particular, both BMP2 and BMP4 present a heparin binding site at their N-terminal end. Therefore both growth factors bind to heparan sulfate, but with different binding kinetics . Their binding to collagen is also different. Indeed it has been demonstrated that the N-terminal prodomain of collagen type IIA binds BMP2 , whereas the C-terminal part of collagen IV binds BMP4 , suggesting that in nature BMP2 and BMP4 are differently distributed in tissues since they interact differently to extracellular matrix components.