 However, others have argued that a rate-limiting slow step further downstream of the process such as slow immobilization and/or slow degradation of Dpp itself could account for the observed differences in diffusion.Single molecules of Dpp have been tracked using fluorescence correlation spectroscopy (FCS), showing that 65% of Dpp molecules diffuse rapidly (consistent with the free diffusion model) and 35% diffuse slowly (consistent with Dpp bound to receptors or glypicans).The restricted diffusion model includes the effects of cell packing geometry and interactions with the extracellular matrix via binding events with receptors such as Tkv and the heparin sulfate proteoglycans "dally" and "dally-like".