CD33 and acute lymphoblastic leukemia: Since elevated expressions of CD22 (Siglec-2) and CD33 (Siglec-3) are described as negative prognosis factors, the clinical use of anti-Siglec-2 and anti-Siglec-3 monoclonal antibodies conjugated with cytotoxins showed therapeutic benefits in patients with acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML), respectively [45,46].