 The transcytosis model assumes Dpp to be transported via repeated rounds of intracellular receptor-mediated endocytosis, with the gradient severity determined by endocytotic sorting of Dpp toward recycling through cells vs degradation.This model was initially based on an initial observation that Dpp could not accumulate across clones where a critical protein called dynamin necessary for endocytosis had been mutated into the "shibire" ("shi") phenotype.However, other experiments showed that Dpp was able to accumulate over "shi" clones, challenging the transcytosis model.