CK5 is a high molecular weight cytokeratin that stains stratified squamous epithelium (e.g. skin, tongue mucosa, etc.), basal cells of the prostate gland, myoepithelial cells in breast tissue, and mesothelial cells/mesothelioma.  Therefore CK5 is an excellent marker, which can be used in a variety of diagnostic applications.

 

CK5 is useful in the diagnosis of squamous cell carcinoma, urothelial carcinoma, mesothelioma, thymic tumors, and skin appendage tumors.  It is also helpful in identifying the myoepithelial layer in breast tissue, and the basal layer in prostate tissue.

 

The combined expression of p63 and CK5 in poorly differentiated tumors is highly suggestive of squamous origin (Kaufmann, O., et al)

CK5 will stain approximately 70-80% of squamous cell carcinomas with a cytoplasmic staining pattern.  p63 is considered slightly more sensitive, but may give varying positivity in cases of lung adenocarcinoma that can be confusing.  Therefore, CK5 and p63 are often used as part of a panel to diagnose squamous cell carcinoma.  Since p63 is a nuclear marker, these two stains can be performed as a double stain on a single slide with one or two different chromogens.

CK5 is an excellent marker for the basal layer, and stains in a similar pattern to 34βE12.  Again, CK5 can be combined with p63, which also stains the basal layer of prostate glands.

 

Bladder

CK5 will stain the myoepithelial layer of breast tissue, but other markers such as smooth muscle myosin and calponin are typically used more commonly.  Basal-like breast carcinomas usually express CK5, especially with the monoclonal CK5 antibody compared to CK5/6 (Bhargava, R, et al).  Metaplastic/sarcomatoid breast carcinomas have also been reported to express CK5.

CK5 will stain the stratified squamous epithelium.  If there are cells in the squamous epithelium, which may be Paget cells or melanocytes, CK5 can be used along with CK7 or Melan A/HMB-45 to differentiate between odd appearing keratinocytes, Paget’s disease, or melanocytes.

There is occasionally discussion as to which is a better marker.  Some feel CK5/6 is a better marker since it has more than one cytokeratin in it’s cocktail.  However, there is a very strong consensus among pathologists who have used both that CK5 performs significantly better than CK5/6, and is therefore, the preferred antibody.  

 

Common expression patterns in carcinoma (Chu, PG, et al).

 

Note:  other references note a much higher + expression pattern in meotheliomas.  This may be dependent upon the mix of epithelial and sarcomatous variants.