The differential distribution of specific proteins in axons or dendrites underlies the specialized functions of these neurites. At least two mechanisms can create a polarized distribution of centrally produced transmembrane proteins: (1) segregation of proteins into distinct vesicles that are specifically targeted to the appropriate domain, and (2) unsorted transport followed by specific endocytosis of inappropriately expressed proteins. This week, Bushlin et al. Suggest that the latter mechanism can be regulated by proteins that associate with endocytic vesicles and determine their cargoes. Knockdown of proteins involved in endocytic vesicle formation, AP180 or clathrin assembly lymphoid myeloid protein (CALM), inhibited axon or dendrite formation, respectively. Knockdown of either protein caused VAMP2—an axonal synaptic vesicle protein that is normally endocytosed from dendrites—to be expressed in all processes, supporting a role in the establishment of polarity. CALM knockdown also reduced surface expression of a secreted protein, suggesting that it may be involved in secretory as well as endocytic pathways.