Wiskott-Aldrich syndrome is an inherited deficiency of T-lymphocyte function and of platelets. The observation in 1981-84 of deficiency and/or defects in Wiskott-Aldrich lymphocytes of the surface molecule sialophorin (CD43) spurred intensive study of this molecule. Sialophorin (CD43) is now known to be a prevalent molecule on most circulating blood cells; it is a transmembrane molecule subject to phosphorylation reactions and capable of intracellular signaling. Oligosaccharides constitute 60% of the molecule. The extracellular region resembles acidic mucin molecules with expanded structure and dense negative charge. The sialophorin (CD43) polypeptide is subject to alternative glycosylation pathways that are cell-specific. CD43 functions in vitro as the receptor of an independent pathway of T-lymphocyte and monocyte activation. CD43 is hypothesized to regulate the survival of blood cells in the circulation. This review covers the distribution, chemistry, cDNA cloning, genetic analysis and functional analysis of CD43, and summarizes recent findings of related defects in Wiskott-Aldrich lymphocytes.