CK5

CK5 (cytokeratin 5), also known as KRT5 (keratin 5), is a key intermediate filament protein encoded by the KRT5 gene. CK5 provides structural support to epithelial cells and serves as a marker for certain epithelial cell lineages, particularly those with basal or squamous differentiation. It is frequently assessed using antibodies that recognize both CK5 and CK6 (CK5/6). However, studies suggest that the diagnostic utility in many contexts is primarily driven by CK5 expression and the inclusion of CK6 may reduce specificity. In normal tissues, CK5 is typically expressed in the cytoplasm of the basal cell layer of stratified epithelia, including the epidermis, urothelium, prostate basal cells, myoepithelial cells of the breast and in mesothelium.

CK5 has an important diagnostic role and is routinely included in immunohistochemical panels to help determine the lineage of carcinomas. It is useful in distinguishing squamous cell carcinomas (typically show diffuse CK5 positivity) and related tumors from adenocarcinomas (negative or only focally positive). In breast and prostate pathology, CK5 is valuable for identifying myoepithelial and basal cell layers in benign and precursor lesions, aiding in the assessment of invasion. In the context of mesothelial lineage, CK5 is a sensitive marker for epithelioid malignant mesothelioma and is consistently used as part of diagnostic antibody panels.

• Lung squamous cell carcinoma (SCC) vs lung adenocarcinoma (AC) CK5 is a key marker in small-biopsy panels for NSCLC subtyping. About 90-95% of SCC shows diffuse CK5 positivity, while AC is CK5 negative or only focally positive. In addition, SCC is also p40 positive (95-100%), while AC being and TTF-1/Napsin A positive (75-85%).
• Malignant mesothelioma CK5 is positive in most epithelioid mesotheliomas but shows lower sensitivity in sarcomatoid cases (up to 30%). It is routinely included in immunohistochemical panels for malignant mesothelioma, where at least two positive mesothelial markers (e.g., CK5, calretinin, WT1, Podoplanin/D2-40) and two positive carcinoma markers (e.g. Claudin-4 and EpCAM) are recommended for differential diagnosis.
• Head & neck squamous cell carcinoma Diffuse CK5 expression supports squamous differentiation in poorly differentiated head and neck primaries and metastases, often used in combination with p40.
• Breast lesions CK5 highlights myoepithelial cells in the basal layer of ducts, aiding distinction between benign proliferative lesions and carcinoma in situ (CK5-positive myoepithelial layer) from invasive lesions (absent myoepithelial layer). CK5 expression also defines the “basal-like” molecular subtype of breast cancer, frequently triple-negative (ER–, PR–, HER2–) and generally associated with poorer prognosis compared to luminal subtypes.
• Prostate lesions CK5 (often used in IHC-double stain with p63) is used to demonstrate basal cells that show positive rection, helping to differentiate benign glands and in-situ lesions (CK5 positive basal cell layer present) from invasive acinar adenocarcinoma (basal cell layer absent).

Tonsil and pancreas can be recommended as positive tissue controls for CK5. In tonsil, virtually all squamous epithelial cells across all layers should display a moderate to strong cytoplasmic staining reaction. In pancreas, scattered cuboidal epithelial cells of the intercalated ducts are expected to show a weak to moderate predominantly cytoplasmic staining reaction, which serves as the critical low-expression control for demonstrating assay analytical sensitivity. Liver is recommended as a negative tissue control, as no reaction should be observed in hepatocytes or bile ducts. It is important to be aware that CK5 expression in pancreas may be affected by autolysis and tissue degradation, which can compromise its reliability as a critical control.