Polarization of epithelial cells is essential for vectorial transport across them. The tight junctions (TJs) that link neighbouring cells and demarcate their apical and basolateral membranes appear to play a crucial role in the establishment of polarity – as do signalling complexes containing the PDZ-domain proteins Par6 and Par3/ASIP, which regulate TJ assembly. Ben Margolis and co-workers now reveal the importance of two components of another TJ-associated complex in this process: the novel junctional protein Crumbs 3(CRB3) and its partner the PDZ-domain protein Pals1 (mammalian homologue of the fly protein Stardust). They show that overexpression of CRB3 disrupts TJ assembly and apicobasal polarity in MDCK epithelial cells and that this requires the C-terminus of CRB3, which interacts with the Pals1 PDZ domain(see p. 2895). The authors also show that a chimeric protein containing the Pals1 PDZ domain and sequences from another PDZ-domain protein (LIN2) functions as a dominant inhibitor of TJ assembly and polarity establishment. Their results thus indicate that the Pals1-CRB3 interaction is critical for establishment of epithelial cell polarity in mammals and underscore the importance of TJs in this process.