Type XXIII collagen is a type II transmembrane collagen found in healthy human and mouse tissues of the lung, cornea, skin, tendon, amnion, and to a lesser extent in the kidney and placenta. Currently, its function is unknown; however, type XXIII collagen is a component of many epithelia and expressed on epithelial cell surfaces, suggesting a role in the formation or maintenance of cell-cell contacts or the polarization of epithelial cells. Type XXIII collagen levels in patient samples have been proposed as a new biomarker of prostate cancer progression and recurrence. Type XXIII collagen has been detected in urine samples of prostate cancer and non-small-cell lung cancer patients and can discriminate between controls and cancer patients, indicating a potential role as a new diagnostic tool in certain types of cancer. There are currently no mutations in humans or mice associated with biological functions; no biomarkers have been put forward.