The establishment of polarity in the embryo is fundamental for the correct development of an organism . The first cleavage of the Caenorhabditis elegans embryo is asymmetric with certain cytoplasmic components being distributed unequally between the daughter cells [2-4]. Using a genetic screen, Kemphues and co-workers have identified six par genes (partition-defective) [5,6], which are involved in the process of asymmetric division. One of these genes encodes a highly conserved protein, PAR-1, which is a serine/threonine kinase that localizes asymmetrically to the posterior part of the zygote and to those blastocysts that give rise to the germ line [7-9]. We reasoned that the mammalian homologue of PAR-1 (mPAR-1) might be involved in the process of polarization of epithelial cells, which consist of apical and basolateral membrane domains. We found that mPAR-1 was expressed in a wide variety of epithelial tissues and cell lines and was associated with the cellular cortex. In polarized epithelial cells, mPAR-1 was asymmetrically localized to the lateral domain. A fusion protein lacking the kinase domain had the same localization as the full-length protein but its prolonged expression acted in a dominant-negative fashion: lateral adhesion of the transfected cells to neighbouring cells was diminished, resulting in the former cells being 'squeezed out' from the monolayer. Moreover, the polarity of these cells was disturbed resulting in mislocalization of E-cadherin. Thus, in the C. elegans embryo and in epithelial cells, polarity appears to be governed by similar mechanisms.