Cytokeratin-5 (CK5) is a protein made by many types of normal cells throughout the body including those in the breast, skin, salivary glands, and prostate. Within these organs, CK5 is commonly seen in specialized squamous cells, basal cells, and myoepithelial cells. Tumors that start from one of these cell types may also produce CK5.
CK5 belongs to a larger family of proteins called cytokeratins, which are responsible for helping cells maintain their structure. Other cytokeratins include cytokeratin-6 (CK6), cytokeratin-7 (CK7), cytokeratin-20 (CK20), cytokeratin-8 (CK8), and cytokeratin-19 (CK19).
Pathologists use a test called immunohistochemistry to ‘see’ CK5 protein inside cells. The CK5 protein is normally found in a part of the cell called the cytoplasm or cell body.
Positive for CK5 means that the CK5 protein was seen inside the cells of interest in the tissue sample. Reactive is another term that pathologists use to describe cells that are producing CK5.
Negative for CK5 means that the CK5 protein was not seen inside the cells of interest in the tissue sample. Non-reactive is another term that pathologists use to describe cells that are not producing CK5.
Pathologists often perform immunohistochemistry for CK5 to determine if the cells that they are seeing under the microscope come from one of the organs or tissues that normally produce this protein. This is particularly important when examining tumour cells that have spread to another part of the body (this type of spread is called metastasis). If the tumour cells are positive for CK5, it suggests that the tumour may be made up of squamous cells, basal cells, or myoepithelial cells and may have started in an organ such as the skin, breast, lung, salivary gland, or prostate. In contrast, if the cells are negative for CK5, it suggests that the tumour is made up of another type of cell or may have come from an organ or tissue that does not normally produce CK5.