CD68 is a 110-kD transmembrane glycoprotein that is highly expressed by human monocytes and tissue macrophages. Holness and Simmons (1993) isolated cDNA clones encoding CD68 by transient expression in COS cells and immunoselection using anti-CD68 monoclonal antibodies. Two types of cDNA clones were isolated, differing by the insertion of 2 short segments in the N-terminal portion of the extracellular domain. The cDNA sequence predicted a type I integral membrane protein of 354 residues with a heavily glycosylated extracellular domain of 298 residues containing 9 potential N-linked glycosylation sites and numerous potential O-linked glycosylation sites. CD68 is a member of a family of hematopoietic mucin-like molecules that includes leukosialin/CD43 and stem cell antigen CD34 .