Cytokeratin-20 (CK20) is a protein that is made mostly by cells in the colon, rectum, bladder, ureters, and urethra. It is also made by a specialized type of cell found in the skin called a Merkle cell.
CK20 belongs to a larger family of proteins called cytokeratins, which are responsible for helping cells maintain their structure. Other cytokeratins include cytokeratin-5 (CK5), cytokeratin-6 (CK6), cytokeratin-7 (CK7), cytokeratin-8 (CK8), and cytokeratin-19 (CK19).
Pathologists use a test called immunohistochemistry to ‘see’ CK20 protein inside cells. The CK20 protein is normally found in a part of the cell called the cytoplasm or cell body.
Positive for CK20 means that the CK20 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 CK20.
Negative for CK20 means that the CK20 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 CK20.
Pathologists often perform immunohistochemistry for CK20 on tumour samples in order to determine the tumour type or to look for specific types of cells within the tumour.
Most tumours from the colon and rectum such as adenocarcinoma and tumours of the bladder such as urothelial carcinoma will produce CK20. A type of skin cancer called Merkle cell carcinoma will also produce this protein as will some tumours from the stomach and pancreas.