The Cytokeratin 5/6 (CK5/6) immunohistochemistry test is a special staining technique used in pathology to detect the presence of CK5 and CK6 proteins in tissue samples. These proteins are intermediate filament proteins found mainly in the cytoplasm of certain epithelial cells, particularly basal and myoepithelial cells.

This test helps in identifying the origin and type of tumor cells, especially in distinguishing different types of carcinomas. CK5/6 is commonly used as a marker of basal-type epithelial differentiation.