CD33 is a 67kD type I transmembrane glycoprotein and a member of the sialoadhesin family of cell surface receptors. It is absent from pluripotent stem cells but appears on myelomonocytic precursors after CD34. It then continues to be expressed on both the myeloid and monocyte lineages, although it is absent on granulocytes. While it has been reported that CD33 can function as a sialic acid-dependent cell adhesion molecule, cells expressing CD33 require desialylation before they can bind cells bearing the appropriate sialoglycoconjugates. This suggests that inhibitory cis interactions may regulate or block any adhesion function.1-4