The clone WM53 selectively binds with a 67kDa glycoprotein, known as CD33 having sialic acid-binding immunoglobulin-like lectin. It belongs to the ITIM (immuno-receptor tyrosine-based inhibitory motif) containing molecules able of recruiting protein tyrosine phosphatases. CD33 is expressed on myeloid progenitors, monocytes, granulocytes, dendritic cells and mast cells. It is absent on platelets, lymphocytes, erythrocytes and hematopoietic stem cells.