The antigen is a single chain transmembrane glycoprotein of molecular mass 67 kDa expressed on monocytes, myeloid progenitor cells and myeloid leukaemias. The antigen has homology with CD22 and myelin associated glycoprotein. CD33 appears very early on myelomonocytic precursors and is expressed in both the myeloid and monocytic cell lineages. CD33 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