CD33 is  a 67kDa type-I transmembrane glycoprotein. It is a cell adhesion molecule expressed mainly on monocytes, granulocytes (weakly), myeloid progenitors and mast cells, but not on lymphocytes, platelets, erythrocytes and hematopoietic stem cells. CD33 antigen is a useful marker for the diagnosis of non lymphoblastic leukemias and also for distinguishing myelogenous leukemia cells from lymphoid leukemia cells. The CD33 antigen is early expressed in the cytoplasm (cyCD33) and then on the cell membrane (mCD33) of the myeloblasts.