 The four main categories of theories behind the formation of the gradient are free diffusion, restricted diffusion, transcytosis, and cytoneme-assisted transport.The free diffusion model assumes Dpp to diffuse freely through the extracellular matrix, degrading via receptor-mediated degradation events.FRAP assays have argued against this model by noting that diffusion of GFP-Dpp does not match that expected of a similarly sized molecule.