CD33 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. The extracellular portion of this receptor contains two immunoglobulin domains (one IgV and one IgC2 domain), placing CD33 within the immunoglobulin superfamily. The intracellular portion of CD33 contains immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that are implicated in inhibition of cellular activity. It is the target of gemtuzumab ozogamicin, an Antibody-drug conjugate for the treatment of patients with acute myeloid leukemia.