Keratin-7 is a member of the keratin gene family. The type II cytokeratins consist of basic or neutral proteins which are arranged in pairs of heterotypic keratin chains coexpressed during differentiation of simple and stratified epithelial tissues. Keratin-7 is found in simple glandular epithelia, and in transitional epithelium. Epithelial cells of the lung and breast both contain keratin-7, but some other glandular epithelia, such as those of the colon and prostate, do not. Because the keratin-7 antigen is found in both healthy and neoplastic cells, antibodies to CK7 can be used in immunohistochemistry to distinguish ovarian and transitional cell carcinomas from colonic and prostate cancers, respectively. Pancreas is recommended as positive tissue control for CK7. Virtually all ductal and intercalated duct epithelial cells must show an at least weak to moderate, distinct cytoplasmic staining reaction, whereas epithelial cells of large ducts should display a strong staining intensity. Acinar epithelial cells should be negative. In general, no staining reaction should be seen in the epithelial cells. Scattered columnar epithelial cells and endothelial cells may show a weak to moderate cytoplasmic staining reaction.