Almost every eukaryotic cell can adopt a polarized subcellular organization. Cells use such polarized organization to migrate, to divide in a coordinate fashion, and to arrange themselves in multicellular structures. An excellent model to study cellular polarity is provided by epithelial cells. Epithelial cells interact with each other and form intercellular cavities (lumens) that are isolated from the environment in which the cells reside. The formation of such intercellular lumens is a fundamental step in the evolution of multicellular organisms as these lumens give rise to tubes that are crucial to ensure the transport of vital fluids and nutrients throughout organ systems of the organism. To put up specific functions at the different extracellular milieus, epithelial cells have segregated their surface into compositionally, functionally, and structurally distinct apical and basolateral domains that face the cavities and underlying tissues, respectively. Instrumental to the establishment of these distinct domains is the intracellular sorting and trafficking of proteins and lipids. Loss of epithelial cell polarity is correlated with diseases such as cancer, and defects in the ability of epithelial cells to correctly sort and/or target apical and basolateral molecules can be directly linked to the pathogenesis of human diseases including a. o. liver disease and microvillus inclusion disease. General aim: to understand how epithelial cells develop and maintain apical-basolateral polarity. Our focus: the epithelial cells apical surface. We want to know what are the cell biological principles that determine the polarized sorting and trafficking of proteins and lipids, and those that determine the development of apical surface domains and lumens.