Cytokeratin 5 is commonly found in the cells on the outermost layer of skin in humans and animals. It is encoded by the KRT5 gene and pairs with the type I keratin K14. Cytokeratin 5 has become an important biomarker for different types of cancer, including mesothelioma, breast cancer and lung cancer. It helps differentiate squamous carcinomas from adenocarcinomas. Pathologists use cytokeratin 5 to distinguish mesothelioma from adenocarcinoma, the most common type of lung cancer. Cytokeratin 5/6 cannot identify mesothelioma on its own. Cytokeratin 5/6 is a positive marker for malignant pleural mesothelioma, found in more than three-fourths of cases. It is also found in certain types of lung cancers and breast cancers. Pathologists use cytokeratin 5/6 to stain cancer tissue samples. Cytokeratin 5/6 immunoreactivity is rarely seen in adenocarcinomas of the lung. If a tumor sample shows strong expression of cytokeratin 5/6, it gives pathologists a hint the tumor is malignant mesothelioma rather than a metastatic adenocarcinoma. However, this marker is not effective for all cell types of mesothelioma. Cytokeratin 5/6 staining is usually weak or negative for sarcomatoid mesothelioma, the least common and hardest-to-treat cell type of the asbestos-related cancer. The marker is also not effective in telling the difference between pleural mesothelioma and squamous cell carcinomas. About 25 to 30 percent of all lung cancers are squamous cell carcinomas.