CD44 cell surface antigen is a 100 kDa type 1 transmembrane glycoprotein widely expressed on human leucocytes, white matter of the brain and by some epithelial cells of the intestine and breast. Several isoforms of CD44 exist, including the predominant CD44H isoform detected in many normal tissues. CD44 is a receptor for hyaluronic acid (HA) and is involved in cell-cell interactions, cell adhesion and migration. CD44 also participates in a wide variety of cellular functions including lymphocyte activation, recirculation and homing. CD44 expression may be up-regulated upon some carcinomas, and it has been speculated that this may be related to metastatic potential. CD44 is expressed by hematopoietic, non-hematopoietic cells, epithelial tissues, and to filopodia in cultured keratinocytes. Further, bone marrow myeloid cells and memory T cells express CD44 at high levels, and peripheral B and T cells can upregulate the expression of CD44 in response to certain stimulatory events. Transcripts for the CD44 gene undergo complex alternative splicing that results in many functionally distinct isoforms, however, the full-length nature of some of these variants have not been determined. Alternative splicing is the basis for the structural and functional diversity of the CD44 protein. Diseases associated with CD44 dysfunction include superficial keratitis and lichen sclerosus. CD44 also may be related to tumor metastasis formation.
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