 A revision of the theory behind the model proposes that endocytosis is not essential for Dpp movement but is involved in Dpp signaling.Dpp fails to move across cells with mutated "dally" and "dally-like", two heparin sulfate proteoglycans (HSPGs) commonly found in the extracellular matrix.As a result, these results suggest that Dpp moves along the cell surface via restricted extracellular diffusion involving "dally" and "dally-like", but the transport of Dpp itself does not rely on transcytosis.