The basic, high molecular weight cytokeratin 5 (58 kDa)is expressed in the superficial, intermediate, and basalcell layers of stratified epithelia, as well as in complexepithelia, transitional epithelia, mesothelial cells, andmesothelioma. Proliferating squamous epithelium alsoexpresses Cytokeratin 6 (56 kD), a high molecularweight, basic type cytokeratin that is frequentlycombined with Cytokeratin 16. Nearly all malignantmesotheliomas have positive cytokeratin 5 and 6antibody results, but lung adeno-carcinomas hardlyever do. Both squamous and undifferentiated large-cellcarcinomas have positive expression of CK 5 and 6. Lessthan 10% of prostate, colon, and breast cancers exhibitpositive staining for this antigen. Antibodies tocytokeratin 5 and 6 have also been effectively employedas a marker of myoepithelial cells in the prostate toidentify cancer.