Robust polarity establishment involves dynamic changes in endo- and exocytic trafficking systems. (A) Transition of a typical in silico WT cell from nonpolarized to a polarized state. Membrane-bound Cdc42-GTP depolarized over the plasma membrane (top left) polarizes to a unique cluster over time (top right). The Cdc42 kymograph (top) shows Cdc42-GTP during polarization. The kymograph in the bottom shows individual endo- and exocytic events over time (x axis) along the cortex (y axis). A tight pole of exocytosis develops (cyan), overlapping the Cdc42-GTP cluster, and is corralled by a ring of endocytosis (red). (B) Random endo- and exocytic distributions in vivo in a nonpolarized cell (left) change to an organized “bull’s-eye pattern” in a polarized cell (right) with a tight exocytic zone surrounded by endocytic vesicles. The endo- and exocytic zones are marked by Abp1-RFP (red) and GFP-Sec4 (cyan), respectively. Kymographs represent the endo- and exocytic profiles along the cortex. Bud emergence occurs at the end of the kymographs. Bars, 2 µm. (C) In silico and in vivo statistical analyses of the coincident tightening of the exocytic (cyan) and endocytic (red) zones as WT cells polarize with the SD between cells represented by error bars.