CD33 is a transmembrane protein that is a member of the sialic acid-binding immunoglobulin-like lactic (Siglec) family. The exact function of CD33 is not known, but it may be involved in cell-to-cell adhesion. It is not expressed on hematopoietic stem cells, but it is expressed on myeloid progenitor cells, monocytes, and granulocytes.