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CD33 or Siglec-3 （sialic acid binding Ig-like lectin 3, SIGLEC3, SIGLEC-3, gp67, p67） is a transmembrane receptor expressed on cells of myeloid lineage. It is usually considered myeloid-specific, but it can also be found on some lymphoid cells. CD33 can be stimulated by any molecule with sialic acid residues such as glycoproteins or glycolipids. Upon binding, the immunoreceptor tyrosine-based inhibition motif （ITIM） of CD33, present on the cytosolic portion of the protein, is phosphorylated and acts as a docking site for Src homology 2 （SH2） domain-containing proteins like SHP phosphatases. This results in a cascade that inhibits phagocytosis in the cell.