The Exo70 protein forms one subunit of the exocyst complex. First discovered in S. cerevisiae, Exo70 and other exocyst proteins have been observed in several other eukaryotes, including humans. In S. cerevisiae, the exocyst complex is involved in the late stages of exocytosis, and is localized at the tip of the bud, the major site of exocytosis in yeast. Exo70 interacts with the Rho3 GTPase. This interaction mediates one of the three known functions of Rho3 in cell polarity: vesicle docking and fusion with the plasma membrane (the other two functions are regulation of actin polarity and transport of exocytic vesicles from the mother cell to the bud). In humans, the functions of Exo70 and the exocyst complex are less well characterized: Exo70 is expressed in several tissues and is thought to also be involved in exocytosis.