The Lin study is based on the fruit fly model (Drosophila). He and his colleagues studied the fruit fly protein called Decapentaplegic (Dpp). Dpp is a morphogen molecule that is similar in structure to the human TGF beta protein.
Dpp functions as a morphogen that is instrumental in forming the wings of a fruit fly. The Lin lab demonstrated that Dpp morphogen molecules are mainly distributed on the cell surface, which suggests that Dpp morphogen moves by an extracellular diffusion mechanism.
To prove this hypothesis, Lin and colleagues blocked cell endocytosis and examined Dpp morphogen movement. They found that inhibiting endocytosis disrupts the cell's ability to transduce Dpp signaling, but does not block Dpp movement across cells.
This experiment allowed researchers to distinguish the role of endocytosis in Dpp signaling activity from Dpp movement, providing evidence that endocytosis is not required for Dpp morphogen movement, albeit it is essential for its signaling activity.