Membrane-associated guanylate kinase homologs (MAGUKs) may play a role in cellular functions preventing tumorigenesis as indicated by the neoplastic phenotype caused by genetic loss of the MAGUK Dlg in Drosophila. To test this possibility, we examined the expression and subcellular localization of the tight junction MAGUK ZO-1, as well as the cell adhesion molecule E-cadherin, in paraffin-embedded breast cancer samples, using immunohistochemistry and confocal microscopy. As expected, normal tissue showed intense staining for ZO-1 at the position of the epithelial tight junctions, but this staining was reduced or lost in 69% of breast cancers analyzed (n = 48). In infiltrating ductal carcinomas (n = 38) there was a reduction in staining in 42% of well differentiated, in 83% of moderately differentiated and 93% of poorly differentiated tumors. ZO-1 staining was positively correlated with tumor differentiation (P = .011) and more specifically with the glandular differentiation of tumors (P = .0019). Reduction in ZO-1 staining was strongly correlated with reduced E-cadherin staining (P = 4.9 x 10(-5)). The results suggest that down-regulation of ZO-1 expression and its failure to accumulate at cell junctions may be causally related to cancer progression. To detect loss of heterozygosity, the ZO-1 gene tjp-1 was mapped relative to other markers in 15q13 and polymorphic markers flanking tjp-1 were identified. The marker D15S1019 showed loss of heterozygosity in 23% of informative tumors (n = 13). Loss of a tjp-1-linked marker suggests that genetic loss may, in some cases, be responsible for the reduction in ZO-1 expression in breast cancer.