Leukosialin also known as sialophorin or CD43 (cluster of differentiation 43) is a transmembrane cell surface protein. Sialophorin (leukosialin) is a major sialoglycoprotein on the surface of human T lymphocytes, monocytes, granulocytes, and some B lymphocytes, which appears to be important for immune function and may be part of a physiologic ligand-receptor complex involved in T-cell activation. Defects in the CD43 molecule are associated with the development of Wiskott–Aldrich syndrome. It also appears in about 25% of intestinal MALTomas. Using immunohistochemistry, CD43 can be demonstrated in the paracortical T-cells of healthy lymph nodes and tonsils; it is also positive in a range of lymphoid and myeloid tumours. Because it stains granulocytes and their precursors, it is an effective marker for myeloid tumours.