Cytokeratin 5/6

Cytokeratin 5/6 (CK5/6) is a group of high molecular weight cytokeratins containing keratin 5 and 6, which can participate in cell morphology maintaining and signal transduction by forming the cytoskeleton. CK5/6 is expressed in squamous epithelium and mesothelium but absent in glandular epithelium, which makes it become an effective indicator for distinguishing squamous cell carcinoma, mesothelioma, and adenocarcinoma. For example, its positivity rate in lung squamous cell carcinoma and pleural mesothelioma is significantly higher than that in lung adenocarcinoma. Moreover, high CK5/6 expression tumors have higher malignancy, which may be because CK5/6 can participate in tumor proliferation and migration by regulating apoptotic pathways and affecting cell morphology. But some research found the CK5/6+ lung adenocarcinoma, which may have squamous epithelial differentiation potential. Meanwhile, CK5/6 is a diagnostic marker of triple negative breast cancer (TNBC), and its up-regulation is related to the shortened overall survival of patients, suggesting that TNBC may originate from basal cells. In addition, CK5/6 is highly expressed in benign prostatic hyperplasia but lowly expressed in prostate cancer, which makes it useful for assisting in identification.