CD33 is putative adhesion molecule of myelomonocytic-derived cells that mediates sialic-acid dependent binding to cells. It preferentially binds to alpha-2,6-linked sialic acid. The sialic acid recognition site may be masked by cis interactions with sialic acids on the same cell surface. In the immune response, CD33 may act as an inhibitory receptor upon ligand induced tyrosine phosphorylation by recruiting cytoplasmic phosphatase(s) via their SH2 domain(s) that block signal transduction through dephosphorylation of signaling molecules. This protein induces apoptosis in acute myeloid leukemia (in vitro). Recombinant human CD33 protein, fused to His-tag at N-terminus, was expressed in E.coli.