HTT and Huntington disease: Although further study is needed to fully identify the protein composition of this unique HTT-Rab4 vesicle population and its exact role in synaptogenesis, we propose that endogenous HTT likely functions as a scaffold to link unique Rab4-containing cargo complexes to kinesin-1 and dynein to regulate their bi-directional motility within axons, which could also dictate the cell autonomous mechanisms seen in HD.