CD44 is a glycoprotein receptor that is involved in cell adhesion, cell migration, and other cell-cell interactions. It plays a role in many cellular functions including lymphocyte activation, recirculation, and homing, hematopoiesis, and the progression and metastasis of cancer cells. CD44 exists in several isoforms, and the main isoform is expressed ubiquitously in the cytoplasm and membrane of various cell types throughout the body in normal and diseased tissues including cancers and autoimmune diseases. The absence of the main isoform, or the presence of certain other isoforms in cancer cells, is associated with poor prognosis. In immunohistochemistry, CD44 is a marker for squamous epithelium, and in cancer it can distinguish reactive urothelium from carcinoma in situ. It is positive in colorectal cancer, glioma, oligodendroglioma, prostatic small cell carcinoma, squamous cell and adneosquamous carcinoma. Loss of CD44 is a general marker of poor prognosis in cancer. References: Front Cell Dev Biol. 2017;5:18. doi: 10.3389/fcell.2017.00018; Saudi Med J. 2015;36(3):273-279. doi: 10.15537/smj.2015.3.9622; Arch Immunol Ther Exp (Warsz). 2004;52(1):13-26. PMID: 15053229; Pathol Int. 2012;62(7):463-470. doi: 10.1111/j.1440-1827.2012.02823.x; Recent Pat Inflamm Allergy Drug Discov. 2017;11(2):77-91. doi: 10.2174/1872213X11666170907111858; Stem Cells Transl Med. 2015;4(9):1033-1043. doi: 10.5966/sctm.2015-0048.