Cytokeratin 5/6 (CK5/6), a high molecular weight protein (58 kDa and 56kDa), is expressed in basal cells of squamous and ductal epithelium, myoepithelial cells, and mesothelial cells in normal tissues. It's predominantly found in squamous cell carcinomas, basal cell carcinomas, thymomas, salivary gland tumors, and mesotheliomas, but rarely in lung adenocarcinomas. CK5/6 is primarily used to differentiate mesothelioma and adenocarcinoma, and can aid in distinguishing benign from malignant prostate lesions when combined with 34βE12 and P504S.