CRB3 (Crumbs homologue 3) is an apical transmembrane protein member of the CRB protein family reported to be associated with tight junctions at the apical surface of epithelial cells and to play a role in apical membrane morphogenesis and tight junction regulation. The formation and maintenance of tight junctions is essential for the development of epithelial cell polarity. Two CRB3 isoforms have been described and CRB3 is homologous to the Crumb protein found in Drosophila. CRB3 contains a putative signal peptide in its first 20 amino acids, a putative extracellular site, and a transmembrane domain. The intracellular domain contains a PDZ-binding motif which is involved in the interactions of CRB3 with PARD6A and MPP5. Loss of CRB3 expression in immortal baby mouse kidney (iBMK) cells was reported to render these cells tumorigenic, suggesting an important role for CRB3 function in maintaining epithelial junction formation and apicobasal polarity required for contact-inhibited growth, and to suppress invasion and metastasis.