Apical polarity protein that plays a central role during the epithelial-to-mesenchymal transition (EMT) at gastrulation, when newly specified mesodermal cells move inside the embryo (PubMed:26496195, PubMed:27870829). Acts by promoting cell ingression, the process by which cells leave the epithelial epiblast and move inside the embryo to form a new tissue layer (PubMed:27870829). The anisotropic distribution of CRB2 and MYH10/myosin-IIB at cell edges define which cells will ingress: cells with high apical CRB2 are probably extruded from the epiblast by neighboring cells with high levels of apical MYH10/myosin-IIB (PubMed:27870829). Also required for maintenance of the apical polarity complex during development of the cortex. Interacts (via intracellular domain) with EPB41L5. O-glucosylation is required for localization at the apical plasma membrane (PubMed:26496195). Distributed in a complex anisotropic pattern on apical cell edges: the level of CRB2 on a cell edge is inversely correlated with the level of MYH10/myosin-IIB (PubMed:27870829). In the adult eye, strongly expressed in the outer nuclear layer, containing the cell bodies of the photoreceptor cells, and in the inner nuclear layer, containing the cell bodies of the horizontal, bipolar, amacrine, and Mueller glial cells (PubMed:15851977). Also expressed in some cells in the ganglion cell layer (or may be displaced amacrine cells rather than ganglion cells) (PubMed:15851977). Expressed in early embryonic cells, more specifically in embryonic regions undergoing dramatic rearrangement, such as the developing neuroepithelium which proceeds with neural tube closure, the anterior splitting lateral plate mesoderm that wraps the pericardial cavity and the differentiating somite epithelium (PubMed:22072575). Widely expressed throughout the epiblast and the lateral plate mesoderm at 7 dpc. At 7.5 dpc-7.75 dpc, continues to be expressed in these regions, and expression is also found on the apical side of the embryonic endoderm, and extraembryonic amnion and allantois (PubMed:22072575). At 8-8.5 dpc, expression is also detected in the heart tube, foregut and the apical side of the somite epithelium (PubMed:22072575). Stronger expression is detected on the apical sides of the splitting lateral plate mesoderm, and the apical side of the neural ectoderm at trunk region (PubMed:22072575). Not expressed in the notochord plate or the extra-embryonic endoderm (PubMed:22072575). Expressed in the developing spinal cord, eye, and forebrain at 10.5 dpc (PubMed:17920587). Expressed in the ventricular layers of the developing neural tube along the entire cranial-caudal length, including the anterior forebrain and the posterior spinal cord at 11.5 dpc (PubMed:17920587). Expressed in apical epithelial kidney cells at 15.5 dpc (PubMed:17920587). O-glucosylated by POGLUT1 at Ser-271; consists of an O-glucose trisaccharide, in which the O-glucose is elongated by the addition of two xylose residues (PubMed:26496195). O-glucosylation is required for localization at the plasma membrane (PubMed:26496195). Embryonic lethality due impaired gastrulation (PubMed:22072575, PubMed:27870829). Mesoderm formation is disrupted, and cells do not ingress (PubMed:27870829). Instead, a single layer forms, and the embryo fails to properly establish its body plan, leading to embryonic arrest (PubMed:27870829). Conditional deletion in the developing retina leads to progressive disorganization during late retinal development: retina show progressive thinning of the photoreceptor layer and sites of cellular mislocalization (PubMed:23001562). Conditional deletion in the dorsal telencephalon leads to defects in the maintenance of the apical complex (PubMed:26802325). Belongs to the Crumbs protein family.