Cytokeratin 5 (58 kDa) is a high molecular weight, basic type of cytokeratin expressed in basal, intermediate and superficial cell layers of stratified epithelia as well as transitional epithelia, complex epithelia and in mesothelial cells and mesothelioma. Cytokeratin 6 (56 kDa) is also a high molecular weight, basic type cytokeratin expressed by proliferating squamous epithelium often paired with cytokeratin 16.

CK5/6 has been shown to express in the vast majority of squamous cell carcinoma, basal cell carcinomas, thymomas, salivary gland tumors, and mesothelioma. It rarely reacts with pulmonary adenocarcinomas. Anti-Cytokeratin 5 & 6 is a useful immunohistochemical marker in the identification of mesothelioma and lung squamous cell carcinoma. The detection of cytokeratin 5 in myoepithelial cells may also be used to aid in the determination of breast and prostate malignancies.