During mammary gland formation, epithelial cells become polarized to form a branching network of ducts. In addition, tight junctions assemble at the apical surface of these cells to control the transfer of solutes to the lumen. Writing in Nature Communications, Nagaoka et al. report that cytoplasmic polyadeny-lation element-binding protein (CPEB) determines mammary epithelial cell polarity by localizing the mRNA of a tight junction protein to promote apical tight junction assembly.
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