CD33, also known as Siglec-3 or gp67, is a 67 kDa glycosylated cell surface receptor that mediates cell-cell interactions and maintains immune cells in a resting state. Although CD33 is found on some lymphoid subsets, it is highly expressed on myeloid lineage cells, and is commonly used for the diagnosis of acute myeloid leukemia (AML). CD33 is an established target for therapy, with CD33-positive blasts detected in almost 90 percent of patients presenting with AML.