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LKB1/PAR-4 is essential for the earliest polarization steps in Caenorhabditis elegans embryos and Drosophila oocytes. Although LKB1 (also known as STK11) is sufficient to initiate polarity in a single mammalian intestinal epithelial cell, its necessity in the formation and maintenance of mammalian epithelia remains unclear. To address this, we completely remove LKB1 from mouse embryos by generating maternal-zygotic Lkb1 mutants and find that it is dispensable for polarity and epithelia formation in the early embryo. Instead, loss of Lkb1 leads to the extrusion of cells from blastocyst epithelia that remain alive and can continue to divide. Chimeric analysis shows that Lkb1 is cell-autonomously required to prevent these extrusions. Furthermore, heterozygous loss of Cdh1 exacerbates the number of extrusions per blastocyst, suggesting that LKB1 has a role in regulating adherens junctions in order to prevent extrusion in epithelia. [ABSTRACT FROM AUTHOR]