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.
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.
CD33 is the target of gemtuzumab ozogamicin (trade name: Mylotarg®; Pfizer/Wyeth-Ayerst Laboratories),  an Antibody-drug conjugate for the treatment of patients with acute myeloid leukemia. The drug is a recombinant, humanized anti-CD33 monoclonal antibody (IgG4 κ antibody hP67.6) covalently attached to the cytotoxic antitumor antibiotic calicheamicin (N-acetyl-γ-calicheamicin) via a bifunctional linker (4-(4-acetylphenoxy)butanoic acid). On September 1, 2017, the FDA approved Pfizer's Mylotarg. 
Gemtuzumab ozogamicin was initially approved by the U.S. Food and Drug Administration in 2000. However, during post marketing clinical trials researchers noticed a greater number of deaths in the group of patients who received gemtuzumab ozogamicin compared with those receiving chemotherapy alone. Based on these results, Pfizer voluntarily withdrew gemtuzumab ozogamicin from the market in mid-2010, but was reintroduced to the market in 2017.   
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- Myeloid cell surface antigen CD33 precursor - Homo sapiens (Human)
- Walter RB, Gooley TA, van der Velden VH, Loken MR, van Dongen JJ, Flowers DA, Bernstein ID, Appelbaum FR (May 2007). "CD33 expression and P-glycoprotein-mediated drug efflux inversely correlate and predict clinical outcome in patients with acute myeloid leukemia treated with gemtuzumab ozogamicin monotherapy". Blood. 109 (10): 4168–70. doi:10.1182/blood-2006-09-047399. PMC 1885511. PMID 17227830.
- Calicheamicin (LL-E33288 antibiotics); ADC Review / Journal of Antibody-drug Conjugates (March 20, 2015)
- Gemtuzumab ozogamicin (Mylotarg®) Drug Description; ADC Review / Journal of Antibody-drug Conjugates (July 19, 2015)
- Pfizer Voluntarily Withdraws Cancer Treatment Mylotarg from U.S. Market; FDA Press Release (June 21, 2010)
- Vadastuximab Talirine (SGN CD33a) Drug Description; ADC Review / Journal of Antibody-drug Conjugates (November 23, 2015)